bcm-2xxx-rpi4 kmeta depends on downstream configs

[benmordaunt]

At the moment, meta-virtualization coerces raspberrypi4-64 builds to use the linux-yocto recipe. However, the bcm-2xxx-rpi4 kmeta from yocto-kernel-cache depends on a whole heap of CONFIGs only present in the downstream Linux repository at github.com/raspberrypi/linux. Therefore, you end up with several kconfig warnings for missing CONFIGs. We should just let meta-raspberrypi use its own Linux recipe as it plays nicely with kmeta now anyway.