Enable linux-yocto-dev as the Linux kernel for Xen on the
Raspberry Pi 4, with the bcm-2xxx-rpi4 KMACHINE and KBRANCH.

Set a Raspberry Pi SD card variable to increase the size of the boot
partition to accommodate both the kernel and the Xen hypervisor.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>