imx6: enable XZ_DEC_BCJ and XZ_DEC_ARM

imx6: enable XZ_DEC_BCJ and XZ_DEC_ARM

squashfs is unable to decompress some blocks without that:

[    1.880530] SQUASHFS error: Unable to read page, block bc77a, size 14c50
[    1.887268] SQUASHFS error: Unable to read fragment cache entry [bc77a]
[    1.893889] SQUASHFS error: Unable to read page, block bc77a, size 14c50
[    1.900636] SQUASHFS error: Unable to read fragment cache entry [bc77a]
[    1.907275] SQUASHFS error: Unable to read page, block bc77a, size 14c50
[    1.913994] SQUASHFS error: Unable to read fragment cache entry [bc77a]
[    1.920631] SQUASHFS error: Unable to read page, block bc77a, size 14c50

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Luka Perkov <luka@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38202 3c298f89-4303-0410-b956-a3cf2f4a3e73