LU-3462 ldiskfs: Subsume ldiskfs's build system into lustre

Remove ldiskfs's independant build system (configure, spec
file, etc.) and merge its build into lustre's build system.

Move the ldiskfs/ldiskfs directory up one level.  The extra
directory level is no longer needed without the independant
ldiskfs build system.

Add a make dependency on ldiskfs's "sources" target in
ldiskfs/autoMakefile.am rather than explicitly launching a
separate make instance from the top level autoMakefile.am.

Remove unused "sources" targets in libcfs, lnet, and lustre/obdclass.

Improve the handling of the --disable-ldiskfs option, and make it
interact with other configure options more cleanly.

Change-Id: Id0787f27269e938032e480c7c627327c0adf2672
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/6850
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Brian J. Murrell <brian.murrell@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>