Improve the customizeiso step to allow restarts

Improve the customizeiso step to allow restarts

* Use a touch-file to determine if the bootstrap has been built, clean out
  the entire pkg installation if it has not before continuing.  Leave
  things intact if the bootstrap has already been installed.

* Use defensive umounts and rm's as needed to allow the null mounts and
  package building environment to be recreated without destroying packages
  already installed.

* Allow /usr/pkgsrc to be read-only.  If it is read-only then a null-mount
  will be used for /usr/pkgsrc/distfiles pointing to /usr/release/distfiles.

* Add a bmake check step when building each package and only build packages
  which have not already been installed.