meta-agl-bsp: Add overrides to Kingfisher BSP changes

meta-agl-bsp: Add overrides to Kingfisher BSP changes

As was done in commit 32744858 for the plain h3ulcb/m3ulcb MACHINE
configuration, add rcar-gen3 overrides to the Kingfisher variable
tweaks to avoid unwanted side effects when building for other
MACHINEs when using multiconfigs.  This is needed to be able to
build the KVM demo image for h3ulcb-kf.

Bug-AGL: SPEC-4760

Change-Id: Ie52f0a4a586398875f88e558ef465162c9fdfec1
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28658
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>