- stm32f7 use sectors instead of pages, they go either from 16KB to
128KB, or from 32KB to 25KB. Smaller sectors are at the begining of
the flash. Slots must start at the begining of a sector to not overlap.
- Minimum required RIOBOOT_HDR_LEN or stm32f7 is 0x200
to respect vector table alignment
- Add CPU_FLASH_BASE
