[archiso] Set default SquashFS compressor to XZ

Also change all references from lzma to xz format.
Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

archiso/mkarchiso

@@ -7,7 +7,7 @@ PACCONFIG="/etc/pacman.conf"
 export LABEL="ARCH_$(date +%Y%m)"
 PUBLISHER="Arch Linux <http://www.archlinux.org>"
 APPLICATION="Arch Linux Live/Rescue CD"
-COMPRESSION="gzip"
+COMPRESSION="xz"
 CREATE_DEFAULT="n"
 INSTALL_DIR="arch"
 
@@ -25,9 +25,9 @@ usage ()
     echo "    -L <label>       Set a label for the disk"
     echo "    -P <publisher>   Set a publisher for the disk"
     echo "    -A <application> Set an application name for the disk"
-    echo "    -c <compressor>  Set SquashFS compression type: gzip, lzma or lzo. Default $COMPRESSION"
+    echo "    -c <compressor>  Set SquashFS compression type: gzip, xz or lzo. Default $COMPRESSION"
     echo "                     NOTES:"
-    echo "                            lzma: not officially supported yet by Linux (2.6.36)"
+    echo "                            xz: needs Linux >= 2.6.38"
     echo "                            lzo:  needs Linux >= 2.6.36"
     echo "    -D <install_dir> Set an install_dir. All files will by located here on ISO (except for syslinux)"
     echo "                     Default $INSTALL_DIR"