Looking for FreeBSD u-boot/kernel debugging help (BeagleBone Black)

[Winston Smith]

I'm looking for any wisdom on debugging FreeBSD/u-boot on the BBB.  I
have had some luck with 11-CURRENT, but I think 10-STABLE is probably
a more realistic target.

I did find this while Googling:

https://wiki.freebsd.org/EmbeddedHandbook

But it's not very complete.  I also discovered that if I disconnect my
serial terminal and reconnect it, it seems to bring the FreeBSD kernel
to a debug prompt of sorts -- is there any documentation on this?

Thanks.