fastboot: allow format on devices with small buffers

Formatting large partitions on devices with small transfer buffers did
not work before since format used a strange path through the code to
send data. It now uses the normal path. Also cleaned up a bit. FS
code now lives in a separate file and the custom path for format is
gone.

Change-Id: If4e01cabc2e250b7c02ca7ce8c268e51d49e1529