Fixed compilation of the ps2avrGB keyboard/firmware (resolving #2425) (#2478)

Fixed compilation of the ps2avrGB keyboard/firmware (resolving #2425) (#2478)

* Fixed compilation of the ps2avrGB keyboard/firmware

This commit fixes the silent compilation error for the ps2avrGB
keyboard/firmware. This error was caused by a lacking default
keymap which it did not have because all keyboards based on it were
moved to another directory. I also added the required config.h
options so it's possible to compile it again and (probably)
flash it on a b.mini.

Lastly, I updated the README to reflect the current state.

This commit fixes #2425

* Referenced the pearl in the ps2avrGB REAMDE

Added a reference to the pearl keyboard in the README of the ps2avrGB keyboard as it is originally based on the firmware as well.