init: add exec_reboot_on_failure builtin

Add exec_reboot_on_failure builtin and refactor the VDC commands that
had similar functionality.  These will now also reboot in the case
that the program cannot be found or run for any reason.

Test: boots normally, reboots if command is not found or if command
      returns status '1'

Change-Id: I1c99498c2b741512a50188e1a325c25e9ec8fba0