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

fs/.gitignore

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

fs/Makefile

@@ -99,10 +99,11 @@ $(COBJS): %$(OBJEXT): %.c
 $(BIN):	$(OBJS)
 	$(call ARCHIVE, $@, $(OBJS))
 
-Make.dep: Makefile $(SRCS)
-	$(Q) $(MKDEP) $(DEPPATH) "$(CC)" -- $(CFLAGS) -- $(SRCS) >$@
+.depend: Makefile $(SRCS)
+	$(Q) $(MKDEP) $(DEPPATH) "$(CC)" -- $(CFLAGS) -- $(SRCS) >Make.dep
+	$(Q) touch $@
 
-depend: Make.dep
+depend: .depend
 
 clean:
 	$(call DELFILE, $(BIN))
@@ -110,5 +111,6 @@ clean:
 
 distclean: clean
 	$(call DELFILE, Make.dep)
+	$(call DELFILE, .depend)
 
 -include Make.dep