AGL for Raspberry Pi 4

Hi,

With the release of the new and more powerful Raspberry Pi 4 I think we need to support it in AGL. I have already started working on this. Raspberry Pi 4 is supported in master of meta-raspberrypi and I have work in progress changes for porting it on top of branch thud for meta-raspberrypi: https://github.com/leon-anavi/meta-raspberrypi/tree/thud

My plan is to integrate the required changes in AGL Yocto/OE layers as we did before for the previous versions of Raspberry Pi.

Thanks,
Leon