py: Update and rework build system for including external C modules.

How to use this feature is documented in docs/develop/cmodules.rst.

docs/develop/cmodules.rst

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+MicroPython external C modules
+==============================
+
+When developing modules for use with MicroPython you may find you run into
+limitations with the Python environment, often due to an inability to access
+certain hardware resources or Python speed limitations.
+
+If your limitations can't be resolved with suggestions in :ref:`speed_python`,
+writing some or all of your module in C is a viable option.
+
+If your module is designed to access or work with commonly available
+hardware or libraries please consider implementing it inside the MicroPython
+source tree alongside similar modules and submitting it as a pull request.
+If however you're targeting obscure or proprietary systems it may make
+more sense to keep this external to the main MicroPython repository.
+
+This chapter describes how to compile such external modules into the
+MicroPython executable or firmware image.
+
+
+Structure of an external C module
+---------------------------------
+
+A MicroPython user C module is a directory with the following files:
+
+* ``*.c`` and/or ``*.h`` source code files for your module.
+
+  These will typically include the low level functionality being implemented and
+  the MicroPython binding functions to expose the functions and module(s).
+
+  Currently the best reference for writing these functions/modules is
+  to find similar modules within the MicroPython tree and use them as examples.
+
+* ``micropython.mk`` contains the Makefile fragment for this module.
+
+  ``$(USERMOD_DIR)`` is available in ``micropython.mk`` as the path to your
+  module directory. As it's redefined for each c module, is should be expanded
+  in your ``micropython.mk`` to a local make variable,
+  eg ``EXAMPLE_MOD_DIR := $(USERMOD_DIR)``
+
+  Your ``micropython.mk`` must add your modules C files relative to your
+  expanded copy of ``$(USERMOD_DIR)`` to ``SRC_USERMOD``, eg
+  ``SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c``
+
+  If you have custom ``CFLAGS`` settings or include folders to define, these
+  should be added to ``CFLAGS_USERMOD``.
+
+  See below for full usage example.
+
+
+Basic Example
+-------------
+
+This simple module named ``example`` provides a single function
+``example.add_ints(a, b)`` which adds the two integer args together and returns
+the result.
+
+Directory::
+
+    example/
+    ├── example.c
+    └── micropython.mk
+
+
+``example.c``
+
+.. code-block:: c
+
+    // Include required definitions first.
+    #include "py/obj.h"
+    #include "py/runtime.h"
+    #include "py/builtin.h"
+
+    #define MODULE_EXAMPLE_ENABLED (1)
+
+    // This is the function which will be called from Python as example.add_ints(a, b).
+    STATIC mp_obj_t example_add_ints(mp_obj_t a_obj, mp_obj_tab_obj) {
+        // Extract the ints from the micropython input objects
+        int a = mp_obj_get_int(a_obj);
+        int b = mp_obj_get_int(b_obj);
+
+        // Calculate the addition and convert to MicroPython object.
+        return mp_obj_new_int(a + b);
+    }
+    // Define a Python reference to the function above
+    STATIC MP_DEFINE_CONST_FUN_OBJ_1(example_add_ints_obj, example_add_ints);
+
+    // Define all properties of the example module.
+    // Table entries are key/value pairs of the attribute name (a string)
+    // and the MicroPython object reference.
+    // All identifiers and strings are written as MP_QSTR_xxx and will be
+    // optimized to word-sized integers by the build system (interned strings).
+    STATIC const mp_rom_map_elem_t example_module_globals_table[] = {
+        { MP_ROM_QSTR(MP_QSTR___name__), MP_ROM_QSTR(MP_QSTR_example) },
+        { MP_ROM_QSTR(MP_QSTR_add_ints), MP_ROM_PTR(&example_add_ints_obj) },
+    };
+    STATIC MP_DEFINE_CONST_DICT(example_module_globals, example_module_globals_table);
+
+    // Define module object.
+    const mp_obj_module_t example_user_cmodule = {
+        .base = { &mp_type_module },
+        .globals = (mp_obj_dict_t*)&example_module_globals,
+    };
+
+    // Register the module to make it available in Python
+    MP_REGISTER_MODULE(MP_QSTR_example, example_user_cmodule, MODULE_EXAMPLE_ENABLED);
+
+
+``micropython.mk``
+
+.. code-block:: make
+
+    EXAMPLE_MOD_DIR := $(USERMOD_DIR)
+
+    # Add all C files to SRC_USERMOD.
+    SRC_USERMOD += $(EXAMPLE_MOD_DIR)/example.c
+
+    # We can add our module folder to include paths if needed
+    # This is not actually needed in this example.
+    CFLAGS_USERMOD += -I$(EXAMPLE_MOD_DIR)
+
+
+Compiling the cmodule into MicroPython
+--------------------------------------
+
+To build such a module, compile MicroPython (see `getting started
+<https://github.com/micropython/micropython/wiki/Getting-Started>`_) with an
+extra ``make`` flag named ``USER_C_MODULES`` set to the directory containing
+all modules you want included (not to the module itself). For example:
+
+
+Directory::
+
+    my_project/
+    ├── modules/
+    │   └──example/
+    │       ├──example.c
+    │       └──micropython.mk
+    └── micropython/
+        ├──ports/
+       ... ├──stm32/
+          ...
+
+Building for stm32 port:
+
+.. code-block:: bash
+
+    cd my_project/micropython/ports/stm32
+    make USER_C_MODULES=../../../modules all
+
+
+Module usage in MicroPython
+---------------------------
+
+Once built into your copy of MicroPython, the module implemented
+in ``example.c`` above can now be accessed in Python just
+like any other builtin module, eg
+
+.. code-block:: python
+
+    import example
+    print(example.add_ints(1, 3))
+    # should display 4

docs/develop/index.rst

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+Developing and building MicroPython
+===================================
+
+This chapter describes modules (function and class libraries) which are built
+into MicroPython. There are a few categories of such modules:
+
+This chapter describes some options for extending MicroPython in C. Note
+that it doesn't aim to be a complete guide for developing with MicroPython.
+See the `getting started guide
+<https://github.com/micropython/micropython/wiki/Getting-Started>`_ for further information.
+
+.. toctree::
+   :maxdepth: 1
+
+   cmodules.rst