cpu/kinetis: change ROM_LEN to not use shell arithmetic

Define ROM_LEN with a non arithmetic value as it is evaluated in
`cortexm_common` without a shell context.
The `K` is correctly handled by both the linker and `cortexm_common`.

This fixes issues when `ROM_LEN` is evaluated in
cortexm_common/Makefile.include.

cpu/kinetis/Makefile.include

@@ -13,7 +13,10 @@ LINKER_SCRIPT = kinetis.ld
 ROM_START_ADDR = 0x00000000
 RAM_BASE_ADDR  = 0x20000000
 RAM_START_ADDR = $$(($(RAM_BASE_ADDR)-($(KINETIS_SRAM_L_SIZE) * 1024)))
-ROM_LEN = $$(($(KINETIS_ROMSIZE) * 1024))
+# Define ROM_LEN with a non arithmetic value
+# as it is evaluated in `cortexm_common` without a shell context
+# The `K` is correctly handled by both the linker and `cortexm_common`.
+ROM_LEN = $(KINETIS_ROMSIZE)K
 RAM_LEN = $$(($(KINETIS_RAMSIZE) * 1024))
 
 CFLAGS += \