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/z16/z16f/z16f2800100zcog/src/.gitignore

@@ -1,2 +1,3 @@
 /Make.dep
+/.depend
 /*.obj