sdcard_image-sunxi.bbclass: make sunxi-sdimg image dependent on rootfs type

The rootfs may be using a filesystem type other than ext3 so it should
not be hardcoded.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Merge pull request #48 from net147/sunxi-sdimg-typedep

sdcard_image-sunxi.bbclass: make sunxi-sdimg image dependent on rootfs type