init-live: default to initrd shell if image isn't found

init-live: default to initrd shell if image isn't found

Adds "debugshell" command line parameter for live/install images.

If the init live fails to find and mount a root-fs image,
dumps to a shell after timeout so that the developer can figure
what's wrong.

Timeout defaults to 30 seconds, but it can be changed as param
argument.

Prior art in Ubuntu. Also, leaving a system stale isn't good form.

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>