sdcard_image-sunxi.bbclass: fixed .xz check
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

O-Computers/meta-sunxi,linux-sunxi/meta-sunxi,O-Computers/meta-sunxi,twoerner/meta-sunxi,ebutera/meta-sunxi,jlucius/meta-sunxi,ebutera/meta-sunxi,remahl/meta-sunxi,jlucius/meta-sunxi,net147/meta-sunxi,net147/meta-sunxi,rofehr/meta-sunxi,soderstrom-rikard/meta-sunxi,linux-sunxi/meta-sunxi,rofehr/meta-sunxi,twoerner/meta-sunxi,geomatsi/meta-sunxi,rofehr/meta-sunxi,jlucius/meta-sunxi,soderstrom-rikard/meta-sunxi,linux-sunxi/meta-sunxi,geomatsi/meta-sunxi,O-Computers/meta-sunxi