xz settings

src/freedombone-image-makefile

 ARCHIVE = $(IMAGE).xz
 SIGNATURE = $(ARCHIVE).sig
 OWNER = 1000
-XZ = xz --no-warn --best --verbose --keep
+XZ = xz --no-warn --verbose --keep --threads=0 -3
 SIGN = -gpg --output $(SIGNATURE) --detach-sig $(ARCHIVE)
 
 # settings for `make test`