cortexm_common/ldscript: add _fw_rom_length variable

It will help testing if it is taken into account and for defining for outside
after.

cpu/cortexm_common/ldscripts/cortexm.ld

@@ -21,10 +21,11 @@
  */
 
 _rom_offset = DEFINED( _rom_offset ) ? _rom_offset : 0x0;
+_fw_rom_length = _rom_length - _rom_offset;
 
 MEMORY
 {
-    rom (rx)    : ORIGIN = _rom_start_addr + _rom_offset, LENGTH = _rom_length - _rom_offset
+    rom (rx)    : ORIGIN = _rom_start_addr + _rom_offset, LENGTH = _fw_rom_length
     ram (w!rx)  : ORIGIN = _ram_start_addr,               LENGTH = _ram_length
 }