Revert "Don't generate .depend anymore"

This reverts commit 79af7fbf4e.

Because:

* btashton reported some issues in local builds:

  https://github.com/apache/incubator-nuttx/pull/603#issuecomment-602264860

* this might be related to the current CI breakage:

  > /bin/sh: 1: /__w/incubator-nuttx/incubator-nuttx/nuttx/tools/mkdeps: not found

graphics/.gitignore

@@ -1,4 +1,5 @@
 /Make.dep
+/.depend
 /*.asm
 /*.obj
 /*.rel

graphics/Makefile

@@ -132,10 +132,11 @@ $(BIN): $(OBJS)
 
 mklibgraphics: $(BIN)
 
-Make.dep: Makefile $(SRCS)
-	$(Q) $(MKDEP) $(DEPPATH) "$(CC)" -- $(CFLAGS) -- $(SRCS) >$@
+.depend: gensources Makefile $(SRCS)
+	$(Q) $(MKDEP) $(DEPPATH) "$(CC)" -- $(CFLAGS) -- $(SRCS) >Make.dep
+	$(Q) touch $@
 
-depend: Make.dep
+depend: .depend
 
 clean_context:
 	$(Q) $(MAKE) -C nxglib -f Makefile.devblit distclean TOPDIR=$(TOPDIR) EXTRADEFINES=$(EXTRADEFINES)
@@ -153,5 +154,6 @@ clean:
 
 distclean: clean clean_context
 	$(call DELFILE, Make.dep)
+	$(call DELFILE, .depend)
 
 -include Make.dep

graphics/nxmu/.gitignore

@@ -1,4 +1,5 @@
 Make.dep
+.depend
 *.swp
 *.asm
 *.rel