stm32/README.md: Update supported MCUs, and submodule and mboot use.
stm32/README.md: Update supported MCUs, and submodule and mboot use.

Also mention mpremote as a way to access the REPL.

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

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