In particular, now the mainline kernel can be built on the RPi 1 as well
(so kernelBaseConfig should always be a mainline defconfig from now on).
And RPi 2 users can now use linux_rpi without doing the
`nixpkgs.config.platform = lib.systems.platforms.raspberrypi2;` dance.

Works fine without, and the 'DRM n' is actually preventing the mainline
VC4 driver from building.

Also remove the upstream device trees beforehand so we don't
accidentally end using them if we mess up in postFixup.

Also on the topic of mess ups, make the copy unconditional since they
always seem to be built on all architectures anyway so checking for
their existence just hides typos.