Provide a syncsys target to build /system files

Builds just the files on /system, but not system.img. Like sync,
but just for /system.

Test: m syncsys
Change-Id: Ic5fc60d79d20315a5aca031dff5e2394e58d6b1c
diff --git a/core/Makefile b/core/Makefile
index 2b602b8..c2d186f 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1988,8 +1988,8 @@
 endif
 endif
 
-.PHONY: sync
-sync: $(INTERNAL_SYSTEMIMAGE_FILES)
+.PHONY: sync syncsys
+sync syncsys: $(INTERNAL_SYSTEMIMAGE_FILES)
 
 #######
 ## system tarball