Makefile: Fix a long line in cmd_mkfitimage

Fix this line which is over the limit.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/Makefile b/Makefile
index 888dd1b..93f2dd6 100644
--- a/Makefile
+++ b/Makefile
@@ -999,7 +999,8 @@
 	>$(MKIMAGEOUTPUT) $(if $(KBUILD_VERBOSE:0=), && cat $(MKIMAGEOUTPUT))
 
 quiet_cmd_mkfitimage = MKIMAGE $@
-cmd_mkfitimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -f $(U_BOOT_ITS) -p $(CONFIG_FIT_EXTERNAL_OFFSET) $@\
+cmd_mkfitimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) \
+	-f $(U_BOOT_ITS) -p $(CONFIG_FIT_EXTERNAL_OFFSET) $@ \
 	>$(MKIMAGEOUTPUT) $(if $(KBUILD_VERBOSE:0=), && cat $(MKIMAGEOUTPUT))
 
 quiet_cmd_cat = CAT     $@