Move stand/ofw/libofw to stand/libofw.
Since rS330365, there has been no particular reason for libofw to be in a
subdirectory of ofw. Move libofw up a level to make it fit in better with
the other top level libraries.
Also add a LIBOFWSRC to stand/defs.mk to match what all the other
libraries are doing.
(cherry picked from commit 475008d6ca47ccb2b4baca59a37421d95916d2ba)