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/microblaze_config.mk b/microblaze_config.mk
index e44c79e..68e7e21 100644
--- a/microblaze_config.mk
+++ b/microblaze_config.mk
@@ -24,4 +24,6 @@
 # MA 02111-1307 USA
 #
 
+CROSS_COMPILE ?= mb-
+
 PLATFORM_CPPFLAGS += -ffixed-r31 -D__microblaze__