MICROPY_PY_STM_CONST defaults to 1 if MICROPY_PY_STM is set. Overriding to
0 disables the named register peripheral constants being including in the
stm32 module.
This saves about 7.5KB of code size for the STM32WL55, which is significant
as this SoC doesn't have a lot of flash.
Signed-off-by: Angus Gratton <angus@redyak.com.au>