If you do some soldering, you should be able to add a sort of "power button" functionality by connecting a transistor controlled by the Arduino instead of a button.
Here is an article saying how to add a power button but switch out the button for a transistor and it should work I believe: https://howchoo.com/g/mwnlytk3zmm/how-to-add-a-power-button-...
The easiest solution to your problem is to connect your MFRC522 to an Arduino nano each. Then you can talk between the raspberry and the 3 arduino using I²C.
Moreover you are able to stack 256 RFID readers using this technique.
I switched the default shell to /bin/sh, essentially the variable BR2_SYSTEM_BIN_SH="bash" from busybox' default shell and I got the login prompt upon boot.
With make menuconfig, it was available under System Configuration > /bin/sh
I made some other changes as well, but I am quite sure this is what fixed it. I will accept my answer when I am ...