Config: Disable XZ parallelism by default

Config: Disable XZ parallelism by default

Exporting XZ_OPT caused that every time xz was called, it automatically
enabled parallelism. The make systemm also launches multiple processes
at the same time to use more processor cores at the same time.

The combination of this causes memory exhaustion even on large systems
and has no performance gain. Therefore this is disabled by default
and only enabled where we need it which is already the case.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>