cpu/cortexm_common/ldscripts: add common linker script for cortexm family

cpu/cortexm_common/ldscripts/cortexm.ld

+/*
+ * Copyright (C) 2017 Inria
+ *
+ * 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
+ * directory for more details.
+ */
+
+/**
+ * @addtogroup      cpu_cortexm_common
+ * @{
+ *
+ * @file
+ * @brief           Memory definitions for the Cortex-M family
+ *
+ * @author          Francisco Acosta <francisco.acosta@inria.fr>
+ *
+ * @}
+ */
+
+_boot_offset = DEFINED( _rom_offset ) ? _rom_offset : 0x0 ;
+
+MEMORY
+{
+    rom (rx)    : ORIGIN = _rom_start_addr + _boot_offset, LENGTH = _rom_length - _boot_offset
+    ram (rwx)   : ORIGIN = _ram_start_addr,                LENGTH = _ram_length
+}
+
+INCLUDE cortexm_base.ld