sunxi: make sdcard image with squashfs as rootfs

sunxi: make sdcard image with squashfs as rootfs

Two things that need to be noted

 - There is no partition type id allocated for squashfs yet.  In the
   case of sunxi, any non-zero value should work and we keep it 83 (the
   value for ext4)
 - Remaining spare space within the rootfs partition, not the entire
   sdcard space will be formated as either f2fs or ext4 and mounted as
   overlay to serve the role of rootfs_data.

Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>