Introduce CONFIG_FIT_EXTERNAL_OFFSET

Introduce CONFIG_FIT_EXTERNAL_OFFSET to give user a choice to choose
where to put the external data.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/Makefile b/Makefile
index 5683714..19d7343 100644
--- a/Makefile
+++ b/Makefile
@@ -894,7 +894,7 @@
 	>$(MKIMAGEOUTPUT) $(if $(KBUILD_VERBOSE:0=), && cat $(MKIMAGEOUTPUT))
 
 quiet_cmd_mkfitimage = MKIMAGE $@
-cmd_mkfitimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -f $(U_BOOT_ITS) -E $@ \
+cmd_mkfitimage = $(objtree)/tools/mkimage $(MKIMAGEFLAGS_$(@F)) -f $(U_BOOT_ITS) -E $@ -p $(CONFIG_FIT_EXTERNAL_OFFSET)\
 	>$(MKIMAGEOUTPUT) $(if $(KBUILD_VERBOSE:0=), && cat $(MKIMAGEOUTPUT))
 
 quiet_cmd_cat = CAT     $@