sdcard_image-sunxi.bbclass: adding ext3 dependency (resolves #46)

Without this explicit dependency,
a race condition would be present between the generation of the sd image
and the generation of the ext3 file,
which leads to corrupt SD images in some circumstances.

Based on meta-raspberrypi commit https://github.com/djwillis/meta-raspberrypi/commit/a760ea5fa293a958ebcf1e5af0e6aa4518de4639