cortexm_common/ldscript: allow defining FW_ROM_SIZE

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