mimxrt: Add flash storage support with VFS and littlefs filesystem.

This commit adds full support for a filesystem on all boards, with a block
device object mimxrt.Flash() and uos.VfsLfs2 enabled.

Main changes are:
- Refactoring of linker scripts to accomodate reserved area for VFS.  VFS
  will take up most of the available flash. 1M is reserved for code.  9K is
  reserved for flash configuration, interrupts, etc.
- Addition of _boot.py with filesystem init code, called from main.c.
- Definition of the mimxrt module with a Flash class in modmimxrt.[ch].
- Implementation of a flash driver class in mimxrt_flash.c.  All flashing
  related functions are stored in ITCM RAM.
- Addition of the uos module with filesystem functions.
- Implementation of uos.urandom() for the sake of completeness of the uos
  module.

It uses sample code from CircuitPython supplied under MIT license, which
uses the NXP SDK example code.

Done in collaboration with Philipp Ebensberger aka @alphaFred who
contributed the essential part to enable writing to flash while code is
executing, among other things.

ports/mimxrt/modules/_boot.py

@@ -0,0 +1,15 @@
+# _boot.py
+# Try to mount the filesystem, and format the flash if it doesn't exist.
+# Note: the flash requires the programming size to be aligned to 256 bytes.
+
+import os
+import mimxrt
+
+bdev = mimxrt.Flash()
+
+try:
+    vfs = os.VfsLfs2(bdev, progsize=256)
+except:
+    os.VfsLfs2.mkfs(bdev, progsize=256)
+    vfs = os.VfsLfs2(bdev, progsize=256)
+os.mount(vfs, "/")