add initial raspberry pi b support

[colin-foster-in-advantage]

Add support for the original Raspberry Pi B model.

I had a really old Pi B lying around and decided to try my luck and learn a little about RAUC / Buildroot.

Since it wasn't "trivial" to get up and running, I figured it might be worth offering this as a PR.

The main differences I learned about the hard way:

• U-boot needed bootz instead of booti for 32-bit
• bootcode.bin is needed in the boot partition
• getty needs to run on tty1 instead of "console"

Tested with a 4GB SD card (that probably also dates back to 2011) and buildroot 2022.05.1

[cdsteinkuehler]

Many thanks!