Speed up by doing a pull and update when possible. Added stopwatch calls to identify where the time is spent. Re-organised to use a parameterised target to do the serious work.

Removed drivce space check for platform builds, as there's already one in the FBF anyway, and platform builds are not generally expected to run concurrently (which was the rationale for the check in the bootstrap.)

In letters file allow timestamp=0 to have the drive left alone