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>
diff --git a/arch/m68k/config.mk b/arch/m68k/config.mk
index 79ae298..33b3d51 100644
--- a/arch/m68k/config.mk
+++ b/arch/m68k/config.mk
@@ -5,7 +5,9 @@
 # SPDX-License-Identifier:	GPL-2.0+
 #
 
-CROSS_COMPILE ?= m68k-elf-
+ifeq ($(CROSS_COMPILE),)
+CROSS_COMPILE := m68k-elf-
+endif
 
 CONFIG_STANDALONE_LOAD_ADDR ?= 0x20000