[issue1332] DFBSD 2.2 - Booting usbcdrom/usbsticks on thinkpad hangs on "BTX Halted"

[Jordan Gordeev]

I've adapted some fixes from FreeBSD for the booting issues that result 
in "BTX halted".
My changes are available at 
git://leaf.dragonflybsd.org/~smtms/dragonfly.git, branch "btxhalted".
*Everybody* should test that these changes don't break booting for them.
Additionally, people that have seen "BTX Halted" on some of their 
computers should test to confirm that the issue is fixed.
What the changes do basically: make the BTX switch the processor to real 
mode before making BIOS calls and then go back to protected mode. The 
previous practice relied on BIOS code that is not exercised by common 
operating systems, and thus not tested during BIOS development.
Some side effects of the changes: the BTX kernel size is now reduced to 
690 bytes. It was 780 bytes previously.
boot2 is now 224 bytes smaller, leaving 296 bytes available; boot2_64 
(for 64-bit disklabels) is 228 bytes smaller, leaving 4904 bytes available.
If somebody decides to commit these changes, please squash the commits.