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

cpu/cortexm_common/Makefile.include

@@ -21,3 +21,5 @@ endif
 
 # ROM_OFFSET: offset in rom to start linking, allows supporting a bootloader
 LINKFLAGS += $(if $(ROM_OFFSET),$(LINKFLAGPREFIX)--defsym=_rom_offset=$(ROM_OFFSET))
+# FW_ROM_LEN: rom length to use for firmware linking. Allows linking only in a section of the rom.
+LINKFLAGS += $(if $(FW_ROM_LEN),$(LINKFLAGPREFIX)--defsym=_fw_rom_length=$(FW_ROM_LEN))

cpu/cortexm_common/ldscripts/cortexm.ld

@@ -21,7 +21,9 @@
  */
 
 _rom_offset = DEFINED( _rom_offset ) ? _rom_offset : 0x0;
-_fw_rom_length = _rom_length - _rom_offset;
+_fw_rom_length = DEFINED( _fw_rom_length ) ? _fw_rom_length : _rom_length - _rom_offset;
+
+ASSERT((_fw_rom_length <= _rom_length - _rom_offset), "Specified firmware size does not fit in ROM");
 
 MEMORY
 {