pi4: pin kernel to 4.19 prior to the YP 3.1.4 uprev

pi4: pin kernel to 4.19 prior to the YP 3.1.4 uprev

The uprev to YP 'dunfell' 3.1.4 would change the default kernel used by the pi4.
This has an effect on the DTBo's used for booting and in CI.

One effect is that create-combined-dtb won't no longer build.
Also we might have to change away from fkms on kernel 5.4 .

Bug-AGL: SPEC-3702
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Change-Id: If7353c1b9fde4ef4fe6456ce969a37e2734c5a79
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25716