Cape compatibility layer for BeagleBone Black and BeagleBone AI

Created by: lorforlinux

@jadonk @RobertCNelson I have merged my compatibility branch into this v4.19.x-ti-overlays branch. I will push the compatible overlays from my bb.org-overlays repo in the src/arm/overlays folder of this repo soon :)

Thanks to @RobertCNelson for the uBoot patch which allows us to plug and play with the capes on BBAI just like on BBBWL/BBB. The patch also allows us to give just the name of the overlay in uEnv.txt and uBoot will try to find it first under /boot/dtb/<uname-r>/overlays and then /lib/firmware.