A board can now define the following linker symbols to configure its flash
storage layout:
_micropy_hw_internal_flash_storage_start
_micropy_hw_internal_flash_storage_end
_micropy_hw_internal_flash_storage_ram_cache_start
_micropy_hw_internal_flash_storage_ram_cache_end
And optionally have a second flash segment by configuring
MICROPY_HW_ENABLE_INTERNAL_FLASH_STORAGE_SEGMENT2 to 1 and defining:
_micropy_hw_internal_flash_storage2_start
_micropy_hw_internal_flash_storage2_end
Signed-off-by: Damien George <damien@micropython.org>