I know it has been a while since you posted this, but i just needed to do the same kind of thing for u-boot for an i.MX8MP. It is very possible that what needs to change for the version of u-boot that comes with a beagle bone blue is different from what needs to change with a i.MX8MP, but for what it is worth I posted what I needed to change to my version of u-boot here: How to change the serial console that u-boot uses?