Extracts EFI firmware installer pkg from High Sierra installer
The original version of this script was from https://github.com/grahamgilbert/imagr/wiki/High-Sierra-Notes
This version includes a solution to get round the fact that the High Sierra 10.13.3 and later installers no longer include the update script previously included.
It has also been re-written to use munkipkg instead of pkgutil and pkgbuild as pkgbuild did not produce an installer package that worked under newer High Sierra versions.