Enable ccache for u-boot compile

Just needed to pass in ccache as a prefix to the CROSS_TARGET variable.

Signed-off-by: Martin Hicks <mort@bork.org>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/u-boot/u-boot.mk

@@ -130,7 +130,7 @@ $(U_BOOT_DIR)/$(U_BOOT_BIN): $(U_BOOT_DIR)/.header_modified
 		CFLAGS="$(TARGET_CFLAGS)" \
 		LDFLAGS="$(TARGET_LDFLAGS)" \
 		$(U_BOOT_CONFIGURE_OPTS) \
-		$(MAKE) CROSS_COMPILE="$(TARGET_CROSS)" ARCH=$(U_BOOT_ARCH) \
+		$(MAKE) CROSS_COMPILE="$(CCACHE) $(TARGET_CROSS)" ARCH=$(U_BOOT_ARCH) \
 		$(U_BOOT_MAKE_OPT) -C $(U_BOOT_DIR)
 
 # Copy the result to the images/ directory