Implement 'xD' filesystem support

[hamishcoleman]

After video conference discussion, we have decided that the path forwards for non-ramdisk support should first consider how to work with Raspberry Pi hardware and only secondly consider how to implement some kind of 'factory default' rollback.

Thus, we will be implementing generic ext4 sdcard filesystem image support.

(See also #36 and #48 for older related tickets)

[benhylau]

To add some discussed implementation details, the partitions may look like this as mentioned in other tickets:

--------------------------------
boot
--------------------------------
swap
--------------------------------
ext4 (later maybe butterfs)
--------------------------------

Using butterfs in place of ext4 will allow for rollback to factory default via fs snapshotting, but that requires kernel recompile.