mmc: Add support for erase command

Partitions on the eMMC card can be erased using the
command "fastboot erase <name of the partition>".

The size of the erasable unit is the erase group size,
which is read from the CSD register. The partitions
are erased in terms of the erase group size. Any
overflow beyond the erase group size is not erased.

Change-Id: I87e56c192c706360a99883f119c7e3db5f388c7f
4 files changed