build-pac.sh: Save snapshot of tree

  Add default snapshot and option to restore snapshot sources
  Upstream commits routinely break a previously successful build.
  This commit easy restoration of the last build tree

Change-Id: I091947e2ebab1c322381601bbff4e5ba41340857
Merge "build-pac.sh: Save snapshot of tree" into pac-4.4
Merge pull request #5 from PAC-man/pac-4.4

Merge latest commits