Updates .firmware_revision when revision set at CLI

When revision was specified at the command line,
$FW_PATH/.firmware_updating wasn't updated to reflect the change.
Now $FW_PATH/.firmware_updating is always updated, so idempotency is
achieved when using revision Id's at the command line.
Fixes #84
Merge pull request #85 from leucos/fix-#84

Updates .firmware_revision when revision set at CLI