Add home partition support

- Split partition table into 4G and 8G versions, that define a home
partition.
- Generate real UUIDs for partitions, and convert fstab to use them
instead of fixed block device IDs.
- Update the flash script to choose a partition table, in the same way
we choose BL2.
- Add a -H flag, to control erasing -- if specified, we erase the whole
mmc0, otherwise, we only flash over existing partitions.

Change-Id: I7e7db994c5484c577977059003a5c3a9d857f60b