sdcard_image-sunxi.bbclass: fixed .xz check

.xz check was relying on a bash check that was not working,
returning "]] : not found" error.
Replaced that check with an sh-compatible check,
from https://github.com/djwillis/meta-raspberrypi/blob/master/classes/sdcard_image-rpi.bbclass
Merge pull request #13 from KristofRobot/xz-fix

sdcard_image-sunxi.bbclass: fixed .xz check