firmware: finalise mtree health check
firmware: finalise mtree health check

Not what I was expecting from a base tool like mtree...

* Missing files are not seen as an error condition, so
  remove our ignored files (they show up as nonexistent)
  and then count how many files are missing.  If there
  is at least a missing file show the output and don't
  say everything is swell.

* Since we prefilter MTREE_OUT on a real error just show
  the output since it's likely there's an error to see.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core