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

syscall/.gitignore

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

syscall/Makefile

@@ -79,11 +79,12 @@ $(BIN1): $(PROXY_OBJS)
 $(BIN2): $(STUB_OBJS)
 	$(call ARCHIVE, $@, $(STUB_OBJS))
 
-Make.dep: Makefile $(SRCS)
+.depend: Makefile $(SRCS)
 	$(Q) $(MKDEP) $(ROOTDEPPATH) $(PROXYDEPPATH) $(STUBDEPPATH) \
-	  "$(CC)" -- $(CFLAGS) -- $(SRCS) >$@
+	  "$(CC)" -- $(CFLAGS) -- $(SRCS) >Make.dep
+	$(Q) touch $@
 
-depend: Make.dep
+depend: .depend
 
 .context: syscall.csv
 	$(Q) $(MAKE) -C $(TOPDIR)$(DELIM)tools -f Makefile.host mksyscall
@@ -105,6 +106,7 @@ endif
 distclean: clean
 	$(call DELFILE, .context)
 	$(call DELFILE, Make.dep)
+	$(call DELFILE, .depend)
 	$(call DELFILE, proxies$(DELIM)*.c)
 	$(call DELFILE, stubs$(DELIM)*.c)