cortexm_common/ldscript: re-use _rom_offset variable name

Inspired by kaspar030 version to removing the new _boot_offset variable.

https://github.com/kaspar030/RIOT/blob/cbf324a66d0c925dd02eca290e17c350945e97b2/cpu/cortexm_common/ldscripts/cortexm.ld

cpu/cortexm_common/ldscripts/cortexm.ld

 /*
  * Copyright (C) 2017 Inria
+ *               2018 Freie Universität Berlin
  *
  * This file is subject to the terms and conditions of the GNU Lesser
  * General Public License v2.1. See the file LICENSE in the top level
@@ -14,16 +15,17 @@
  * @brief           Memory definitions for the Cortex-M family
  *
  * @author          Francisco Acosta <francisco.acosta@inria.fr>
+ *                  Gaëtan Harter <gaetan.harter@inria.fr>
  *
  * @}
  */
 
-_boot_offset = DEFINED( _rom_offset ) ? _rom_offset : 0x0 ;
+_rom_offset = DEFINED( _rom_offset ) ? _rom_offset : 0x0;
 
 MEMORY
 {
-    rom (rx)    : ORIGIN = _rom_start_addr + _boot_offset, LENGTH = _rom_length - _boot_offset
-    ram (w!rx)  : ORIGIN = _ram_start_addr,                LENGTH = _ram_length
+    rom (rx)    : ORIGIN = _rom_start_addr + _rom_offset, LENGTH = _rom_length - _rom_offset
+    ram (w!rx)  : ORIGIN = _ram_start_addr,               LENGTH = _ram_length
 }
 
 INCLUDE cortexm_base.ld