cortexm_common: refactor the definition test

The variables should all always be defined.

cpu/cortexm_common/Makefile.include

@@ -6,7 +6,13 @@ ifneq (,$(ROM_OFFSET))
   LINKFLAGS += $(LINKFLAGPREFIX)--defsym=_rom_offset=$(ROM_OFFSET)
 endif
 
+# All variables must be defined in the CPU configuration when using the common
+# `ldscripts/cortexm.ld`
 ifneq (,$(ROM_START_ADDR)$(RAM_START_ADDR)$(ROM_LEN)$(RAM_LEN))
+  $(if $(ROM_START_ADDR),,$(error ROM_START_ADDR is not defined))
+  $(if $(RAM_START_ADDR),,$(error RAM_START_ADDR is not defined))
+  $(if $(ROM_LEN),,$(error ROM_LEN is not defined))
+  $(if $(RAM_LEN),,$(error RAM_LEN is not defined))
   LINKFLAGS += $(LINKFLAGPREFIX)--defsym=_rom_start_addr=$(ROM_START_ADDR)
   LINKFLAGS += $(LINKFLAGPREFIX)--defsym=_ram_start_addr=$(RAM_START_ADDR)
   LINKFLAGS += $(LINKFLAGPREFIX)--defsym=_rom_length=$(ROM_LEN)