[syslinux-iso] Change initramfs compression lzma -> xz

Needs mkinitcpio >= 0.6.9-1 (otherwise does not work with passed options to xz).
Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

configs/syslinux-iso/mkinitcpio.conf

@@ -3,4 +3,4 @@ MODULES=""
 BINARIES=""
 FILES=""
 HOOKS="base udev memdisk archiso archiso_pxe_nbd archiso_loop_mnt pata scsi sata usb fw pcmcia filesystems usbinput"
-COMPRESSION="lzma"
+COMPRESSION="xz"