The syntax
CROSS_COMIPLE ?= <cross_compiler_prefix>
does not work because config.mk is parsed after
exporting CROSS_COMPILE.
Like Linux Kernel's arch/$(ARCH)/Makefile,
we must write as follows:
ifeq ($(CROSS_COMPILE),)
CROSS_COMPILE := <cross_compiler_prefix>
endif
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>