barebox: use ccache if enabled

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/barebox/barebox.mk

@@ -19,7 +19,7 @@ else
 BAREBOX_ARCH=$(KERNEL_ARCH)
 endif
 
-BAREBOX_MAKE_FLAGS = ARCH=$(BAREBOX_ARCH) CROSS_COMPILE=$(TARGET_CROSS)
+BAREBOX_MAKE_FLAGS = ARCH=$(BAREBOX_ARCH) CROSS_COMPILE="$(CCACHE) $(TARGET_CROSS)"
 
 $(DL_DIR)/$(BAREBOX_SOURCE):
 	 $(call DOWNLOAD,$(BAREBOX_SITE),$(BAREBOX_SOURCE))