serial console "detection" during boot

[Andriy Gapon]

Here is an update on the changes:
http://people.freebsd.org/~avg/boot-comconsole.diff

Please note that the file is actually a patchset that consists of three
individual changes:
- a generic change in common boot code
- libi386 comconsole change
- BTX and boot2-ish comconsole change

All the code is lightly tested.
As I am not an expert in the assembly code and also because boot2 is quite
size-sensitive I would like to ask for a special attention to the last change.

Thank you!!

-- 
Andriy Gapon