LU-363 don't cache BUILD for reuse

LU-363 don't cache BUILD for reuse

Since future builds don't actually need the %_topdir/BUILD subdir,
and it's huge, don't bother caching it.

Increment the toplevel BUILD_GEN to force a rebuild of all of the
kernels.

Signed-off-by: Brian J. Murrell <brian@whamcloud.com>
Change-Id: I6a7a523fdaed2073d719f44a24b556bc4595d4ec
Reviewed-on: http://review.whamcloud.com/863
Tested-by: Hudson
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Michael MacDonald <mjmac@whamcloud.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>