push CROSS_COMPILE out to $(ARCH)_config.mk

Each arch should handle setting a proper default CROSS_COMPILE value in
their own config.mk file rather than having to maintain a large ugly list
in the Makefile.  By using conditional assignment, we don't have to worry
about the variable already being set (env/cmdline/etc...).

The common config.mk file takes care of exporting CROSS_COMPILE already,
and while a few variables (toolchain ones) utilize CROSS_COMPILE before
including the arch config.mk, they do so with deferred assignment.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/m68k_config.mk b/m68k_config.mk
index 12bd27c..f41d1b3 100644
--- a/m68k_config.mk
+++ b/m68k_config.mk
@@ -21,5 +21,7 @@
 # MA 02111-1307 USA
 #
 
+CROSS_COMPILE ?= m68k-elf-
+
 PLATFORM_CPPFLAGS += -DCONFIG_M68K -D__M68K__
 PLATFORM_LDFLAGS  += -n