Cleanup profile when restarting B2G on devices. Fixes issue #543

This is a replacement for #635 with the following differences:
* Also remove the profile while the B2G process is stopped
* Wait for the FTU app to finish loading when starting B2G process

I also moved the binary restart to pull #734 so that it wasn't held up by this pull, but it hasn't landed yet. Whichever lands first will cause the other pull to require changes.