py/mkrules.mk: Add MICROPY_PREVIEW_VERSION_2.

This provides a way to enable features and changes slated for MicroPython
2.x, by running `make MICROPY_PREVIEW_VERSION_2=1`. Also supported for
the cmake ports (except Zephyr).

This is an alternative to having a 2.x development branch (or equivalently,
keeping a 1.x release branch). Any feature or change that needs to be
"hidden" until 2.x can use this flag (either in the Makefile or the
preprocessor).

A good example is changing function arguments or other public API features,
in particular to aid in improving consistency between ports.

When `MICROPY_PREVIEW_VERSION_2` is enabled, the REPL banner is amended to
say "MicroPython (with v2.0 preview) vX.Y.Z", and sys.implementation gets a
new field `_v2` set to `True`.

This work was funded through GitHub Sponsors.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/mkrules.cmake

@@ -15,6 +15,10 @@ set(MICROPY_ROOT_POINTERS_SPLIT "${MICROPY_GENHDR_DIR}/root_pointers.split")
 set(MICROPY_ROOT_POINTERS_COLLECTED "${MICROPY_GENHDR_DIR}/root_pointers.collected")
 set(MICROPY_ROOT_POINTERS "${MICROPY_GENHDR_DIR}/root_pointers.h")
 
+if(NOT MICROPY_PREVIEW_VERSION_2)
+    set(MICROPY_PREVIEW_VERSION_2 0)
+endif()
+
 # Need to do this before extracting MICROPY_CPP_DEF below. Rest of frozen
 # manifest handling is at the end of this file.
 if(MICROPY_FROZEN_MANIFEST)
@@ -24,6 +28,12 @@ if(MICROPY_FROZEN_MANIFEST)
     )
 endif()
 
+if(MICROPY_PREVIEW_VERSION_2)
+    target_compile_definitions(${MICROPY_TARGET} PUBLIC
+        MICROPY_PREVIEW_VERSION_2=\(1\)
+    )
+endif()
+
 # Provide defaults for preprocessor flags if not already defined
 if(NOT MICROPY_CPP_FLAGS)
     get_target_property(MICROPY_CPP_INC ${MICROPY_TARGET} INCLUDE_DIRECTORIES)