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: Ibd3e10c4052ca8012c8fd507be99a3b24e68d71e
Reviewed-on: http://review.whamcloud.com/533


Reviewed-by: Chris Gearing <chris@whamcloud.com>
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Tested-by: Hudson
Reviewed-by: Oleg Drokin <green@whamcloud.com>