Got to a prompt!
I followed the instructions here: Building a initramfs image
and built the same type of image
Then from uboot I did
ar7240> tftpboot 0x82000000 openwrt-ar71xx-generic-ubnt-bullet-m-initramfs-kernel.bin
ar7240> setenv bootargs console=tty0 root=31:03 rootfstype=ramfs init=/init
ar7240> bootm 0x82000000
## Booting image at 82000000 ...
Image Name: MIPS OpenWrt Linux-4.14.109
Created: 2019-04-04 15:09:40 UTC
Image Type: MIPS Linux Kernel Image (lzma compressed)
Data Size: 3534602 Bytes = 3.4 MB
Load Address: 80060000
Entry Point: 80060000
Verifying Checksum at 0x82000040 ...OK
Uncompressing Kernel Image ... OK
Starting kernel ...
[ 0.000000] Linux version 4.14.109 (che@che76) (gcc version 7.4.0 (OpenWrt GCC 7.4.0 r9773-6e060bd62c)) #0 Thu Apr 4 15:09:40 2019
[ 0.000000] bootconsole [early0] enabled
[ 0.000000] CPU0 revision is: 0001974c (MIPS 74Kc)
...
BusyBox v1.30.1 () built-in shell (ash)
_______ ________ __
| |.-----.-----.-----.| | | |.----.| |_
| - || _ | -__| || | | || _|| _|
|_______|| __|_____|__|__||________||__| |____|
|__| W I R E L E S S F R E E D O M
-----------------------------------------------------
OpenWrt SNAPSHOT, r9773-6e060bd62c
-----------------------------------------------------
=== WARNING! =====================================
There is no root password defined on this device!
Use the "passwd" command to set up a new password
in order to prevent unauthorized SSH logins.
--------------------------------------------------
root@OpenWrt:/#
Should I be concerned that there are no detected network adapters? I went through the build config pretty quickly so I'm not sure if I missed anything. I suppose now that I've reached this stage I have a whole net set of docs to go through...