cbmem: Makefile: Allow to override `CC` variable

Now users can use a different compiler from GCC like Clang by for example
doing `CC=clang make`.

Change-Id: I664a36df79f7496a56d89bdb61948b2eda33a6b4
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3082
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/cbmem/Makefile

@@ -19,7 +19,7 @@
 
 PROGRAM = cbmem
 ROOT = ../../src
-CC     = $(CROSS_COMPILE)gcc
+CC     ?= $(CROSS_COMPILE)gcc
 CFLAGS ?= -O2
 CFLAGS += -Wall -Werror
 CPPFLAGS += -iquote $(ROOT)/include -iquote $(ROOT)/src/arch/x86