kbuild: fix CROSS_COMPILE settings in config.mk

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>