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

boards/arm/stm32h7/nucleo-h743zi/kernel/Makefile

@@ -109,7 +109,9 @@ $(TOPDIR)$(DELIM)User.map: nuttx_user.elf
 	$(Q) $(NM) nuttx_user.elf >$(TOPDIR)$(DELIM)User.map
 	$(Q) $(CROSSDEV)size nuttx_user.elf
 
-depend:
+.depend:
+
+depend: .depend
 
 clean:
 	$(call DELFILE, nuttx_user.elf)