Explicitly set number of xz compressor threads

mkrootfs.sh.in

@@ -61,6 +61,7 @@ Options
     -C <file>   Full path to the XBPS configuration file
     -h          Show this help
     -r <repo>   Set XBPS repository (may be set multiple times)
+    -x <num>    Use <num> threads to compress the image (dynamic if unset)
     -V          Show version
 _EOF
 }
@@ -71,12 +72,13 @@ _EOF
 
 # Boilerplate option parsing.  This script supports the bare minimum
 # needed to build an image.
-while getopts "C:c:h:r:V" opt; do
+while getopts "C:c:h:r:x:V" opt; do
     case $opt in
         C) XBPS_CONFFILE="-C $OPTARG";;
         c) XBPS_CACHEDIR="--cachedir=$OPTARG";;
         h) usage; exit 0;;
         r) XBPS_REPOSITORY="$XBPS_REPOSITORY --repository=$OPTARG";;
+        x) COMPRESSOR_THREADS="$OPTARG" ;;
         V) echo "$PROGNAME @@MKLIVE_VERSION@@"; exit 0;;
     esac
 done
@@ -205,7 +207,7 @@ rm -rf "$ROOTFS/var/cache/*" 2>/dev/null
 # Finally we can compress the tarball, the name will include the
 # architecture and the date on which the tarball was built.
 tarball=void-${XBPS_TARGET_ARCH}-ROOTFS-$(date '+%Y%m%d').tar.xz
-run_cmd "tar -cp --posix --xattrs -C $ROOTFS . | xz -T0 -9 > $tarball "
+run_cmd "tar -cp --posix --xattrs -C $ROOTFS . | xz -T${COMPRESSOR_THREADS:-0} -9 > $tarball "
 
 # Now that we have the tarball we don't need the rootfs anymore, so we
 # can get rid of it.