Touch a first-boot.status file:

After running first-boots scripts, both to indicate
that the scripts have completed, and also to record
the exit status.

This is useful for automated testing:
  - tests can wait until first-boot scripts have
    completed before commencing testing
  - tests can abort further testing if first-boot
    scripts have failed.

Change-Id: Idec63de858c15def8c621056232aad45eab225e8