uek-rpm: use multi-threaded xz compression for rpms

uek-rpm: use multi-threaded xz compression for rpms

By default kernel rpms use xz compression for all rpms files and
gzip compression for src.rpms. This commit changes compression type to xz
for all rpms produced, and enables multi-threaded compression for rpms. It
allows to use as many threads as there are CPU cores available.

Orabug: 29323635

Signed-off-by: Alex Burmashev <alexander.burmashev@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>