Many MCUs contain some Backup or Low Power SRAM that is retained'even
in the deepest sleep modes.
In such sleep modes the MCU is essentually turned off with only the RTC
still running.
It can be woken by a GPIO or a RTC alarm. When this happens, a reset is
triggered and the normal startup routine is invoked.
This adds bss & data section for this memory in the linker script.
This allows for structures to be placed in it e.g.:
e.g.:
static uint8_t persistent_buffer[64] __attribute__((section(".backup.bss")));
static uint32_t persistent_counter __attribute__((section(".backup.data"))) = 1234;
Allow defining a specific rom length to use for linking the firmware,
_fw_rom_length, instead of the default configuration to use the whole rom from
_rom_offset to the end.
* Add cortexm_common/Makefile.include FW_ROM_SIZE configuration
* Add an assertion that _fw_rom_length still respects _rom_length
