Added a flash driver for the STM32G4 series. The primary change here is
the addition of stm32g4xxx_flash.c. This file uses the STM32L4 flash
driver as a template. The primary difference is the accounting for dual
banks with different page sizes.
Fixed error while building b-g474e-dpow1/buckboost. It was possible (technically) to have page be used uninitialzied. Changing the if statement to default to using a flash_page_size == 2048 fixes this issue.
df47241a2b