If mboot is built with support for packing (signing/encryption) it needs up
to 32KiB. So for simplicity increase the mboot region to 32KiB
unconditionally for WB55 boards (custom WB55 board configurations can still
provide their own linker scripts to override this).
The default stm32lib remains lib/stm32lib, but it can now be easily
overriden at build time by specifying STM32LIB_DIR, or STM32LIB_CMSIS_DIR
and STM32LIB_HAL_DIR.
Signed-off-by: Damien George <damien@micropython.org>
