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

arch/misoc/src/.gitignore

@@ -1,3 +1,4 @@
+/.depend
 /Make.dep
 /locked.r
 /board

arch/misoc/src/Makefile

@@ -164,14 +164,15 @@ export_startup: board/libboard$(LIBEXT) $(STARTUP_OBJS)
 
 # Dependencies
 
-Make.dep: Makefile chip/Make.defs $(SRCS)
+.depend: Makefile chip/Make.defs $(SRCS)
 ifeq ($(BOARDMAKE),y)
 	$(Q) $(MAKE) -C board TOPDIR="$(TOPDIR)" depend
 endif
 	$(Q) $(MKDEP) --dep-path chip --dep-path common --dep-path $(ARCH_SUBDIR) \
-	 "$(CC)" -- $(CFLAGS) -- $(SRCS) >$@
+	 "$(CC)" -- $(CFLAGS) -- $(SRCS) >Make.dep
+	$(Q) touch $@
 
-depend: Make.dep
+depend: .depend
 
 clean:
 ifeq ($(BOARDMAKE),y)
@@ -185,5 +186,6 @@ ifeq ($(BOARDMAKE),y)
 	$(Q) $(MAKE) -C board TOPDIR="$(TOPDIR)" distclean
 endif
 	$(call DELFILE, Make.dep)
+	$(call DELFILE, .depend)
 
 -include Make.dep