For development, booting through TFTP can be handy. The U-Boot can load and start a kernel through TFTP but the process is a bit annoying. The code tries to do some TRX verification but hangs/fails. Luckily, the function can be patched at runtime to skip the verification. I've made a ckermit script to easily boot it: https://gist.github.com/hberntsen/9bfd7e9c25948c796656fc0ab802d6c9
Just run a TFTP server at 192.168.11.2
If you want to play with the kernel arguments, the way OpenWRT compiles the Linux kernel prevents it from loading them from U-Boot. After changing that through make kernel_menuconfig you can set the bootargs U-Boot environment variable.