Allow erasing mmc0, speed up erase

- Add in the ability to erase the whole MMC, using the name of the
partition table (as we are able to do on LK)
- Modify erase to not use the FASTBOOT_MAX_BLK_WRITE, just erase
everything in one call.
- Erase more than one erase_grp_size at a time: MMC spec allows for
erasing more than one erase group at a time, so erase everything in one
command.

Change-Id: I57f1d531053992c5c654e61f13dbd3c505facf5b