The 5.10 version of the Linux Yocto kernel supports running Xen on the
Raspberry Pi 4, so the previous preference for linux-yocto-dev can be
replaced.

Use the ?= operator to set the kernel preference now since
meta-raspberrypi commit e0b4b8c5d introduced use of the weak default
for kernel preference in that layer to allow this here.

KERNEL_DEVICETREE needs to be set to match the build outputs of the
selected kernel, in this case: "broadcom/bcm2711-rpi-4-b.dtb"

Tested with kernel version:
 SRCREV_machine = "cf5b0320cf4544d3db9ce3ddd6ddb7553a610651"
 SRCREV_meta="031f6c76e488a3563f35258c72ff1de3e25a512e"

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