Implement 'fastboot format' command

Some filesystems (e.g. ext4) require flushing an initial
fs image, right after erasing it the partition is unusable.

Doing erase,flush emptyfs is a little bit scaring so we have a
separate command that performs it as atomic step:

 - get size of partition
 - create an empty filesystem image
 - erase the partition
 - flush empty fs to the partition

This command applicable only for ext4 filesystem and checks the
partition type before formatting it.

Change-Id: Ifa42deaa66c3cb96ff786a73c3fadad92658f395