stm32/mpconfigport.h: Add support for a board to specify root pointers.
stm32/mpconfigport.h: Add support for a board to specify root pointers.

A board can now define MICROPY_BOARD_ROOT_POINTERS to specify any custom
root pointers.

Signed-off-by: Damien George <99e6b749acbfbe2a596df99e91d24d6e1fdbee00@micropython.org>

henriknelson/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,adafruit/circuitpython,bvernoux/micropython,bvernoux/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,henriknelson/micropython,henriknelson/micropython,bvernoux/micropython,adafruit/circuitpython,bvernoux/micropython,henriknelson/micropython