package_rpm: pass XZ_THREADS to rpm

By default RPM uses the number of cores as the number of threads to use,
which can result in quite antisocial memory usage.

As we control the macros for compression anyway, we can pass XZ_THREADS
to limit the number of threads if needed.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>