What is a u-Boot dtb file and how do I use it (BeagleBoard xM)?

Answering my own question, after I figured out what to search for. A "dtb" file contains a Device Tree Blob (or Binary)(nice description here). It's the new(er) way to pass hardware information about the board to the Linux kernel.

It can be loaded into memory and passed to the kernel by u-Boot.

Here are the u-Boot commands I used:

setenv bootargs 'console=tty0 console=ttyO2,115200n8 root=/dev/mmcblk0p2 rootwait rootfstype=ext4 rw'
fatload mmc 0:1 0x80300000 zImage
fatload mmc 0:1 0x815f0000 beagle-xm.dtb
bootz 0x80300000 - 0x815f0000

zImage being the kernel, and beagle-xm.dtb being the device tree blob. I automated the boot process by setting up a "uenvcmd=..." variable in uEnv.txt (with the above in it), but it's a bit ugly and there are probably better ways.

Note that this boots up, and says that it loaded the device tree OK. However, I still have no USB devices or video (as far as I can tell). However, I suspect that may be a different problem.

I am using Ubuntu 14.04 on the beaglebone black, so my setup may be a bit different, but here is how I loaded a custom dtb:

In uEnv.txt, add a line with: dtb=name-of-desired.dtb

The locations searched should be as specified here. For me this was /boot/dtbs/linux-kernel/ where linux-kernel is the name of the loaded version.