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.
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D23000
