Replicant

From the first post:

Development board hardware features

I'll start by a listing of the features this development board provides.
This particular development board is not at all important in this project's context, it is simply the platform I will use for demonstration, so you, the reader, need not worry about not having access to it.

So they seem to be using a custom board.
Though they seem to use a friendlyarm board here:
https://blog.korena.xyz/fsol2/

And they seem to write their own BL1 (to be verified).

from https://blog.korena.xyz/bare-metal/

we have:

BL1.s:
[no license]
[some assembler]