fastboot: support for overriding format fs-type and size

This changes allows overriding the fs-type and size that
are normally returned by the booloader.

This is in preparation for supporting other FSes.

Change-Id: I8d141a0d4d14df9fe84d3b131484e9696fcd8870
Signed-off-by: JP Abgrall <jpa@google.com>