Include meta-arm-toolchain layer when using meta-arm

After recent changes on both the dunfell and master branches of the
meta-arm layer, we now need to also include the meta-arm-toolchain layer
(from the same repository) to allow builds to succeed.

Bug-AGL: SPEC-3732

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Change-Id: I6a3e430edaa87d1f684f83a7211a59c62f120add
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25801
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>