boot2/loader: serial port handling

[Doug Ambrisko]

On Fri, Oct 12, 2012 at 05:13:47PM -0700, Garrett Cooper wrote:
| On Fri, Oct 12, 2012 at 5:09 PM, Xin Li <delphij at delphij.net> wrote:
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| ...
| 
| > Ah I wish I am not this far behind my email backlog.  Yes I think
| > these (241300 and 241301) will solve the problem.
| 
|     Yeah -- forgot about the other one. There's another enhancement
| that would make this even better (apart from maybe having multiple
| primary consoles): setting the primary console if present and having
| fallbacks in the event that the original primary wasn't set or
| configurable; it was a thing that was present in another project I
| worked on with sio that was pretty slick (and I think that there would
| be some parties who wouldn't mind if the same was done with uart(4)).

This concept was objected to when I checked it into sio(4) so I had
to back it out.  Some liked it.  I have ported it to uart(4) since we
need that functionality when we moved to a newer FreeBSD.

Doug A.