tools: give explicit libfdt paths

The current libfdt object rules hard depend implicitly on the .depend file
being correct.  If it isn't, then it is unable to properly compile the
objects.  Give it a full path like all the other implicit rules here so it
will always work in face of .depend issues.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

tools/Makefile

@@ -196,7 +196,7 @@ $(obj)%.o: $(SRCTREE)/common/%.c
 $(obj)%.o: $(SRCTREE)/lib_generic/%.c
 	$(HOSTCC) -g $(HOSTCFLAGS) -c -o $@ $<
 
-$(LIBFDT_OBJS):
+$(obj)%.o: $(SRCTREE)/libfdt/%.c
 	$(HOSTCC) -g $(HOSTCFLAGS_NOPED) -c -o $@ $<
 
 subdirs: