See GitHub. 

1. get the nix package manager, turn on flakes and nix-command

2. type in "nix build". you may need an arm linux machine to build the kernel, or will need a VM

I will probably post binaries to make this easier on the GitHub releases page so you can skip this step and download instead/

An SD card should have come with your RG552 (perhaps 2?)

The stock uboot already gives the sd cards priority

If you're OK with the slower speeds associated with booting off the SD card, skip tihs step

Once you overwrite the eMMC, you will lose the original android OS that shipped with your console. IMO, you're not missing out. 

Plug in a keyboard if you have one

Get the same image on your rg552 (either copy it on, flash another SD card, or connect to the network and download it). 

dd it to the emmc partition. remove the sd cards to make sure it boots off eMMC

Add/remove packages from the flake, and build your own closure! 

Currently there's no grub support, but rebuild-switch will still update the extlinux file appropriately