Add AGL reference hardware feature

Add AGL reference hardware feature

Add a feature template to enable AGL H3-based reference hardware
support.  A feature is used as opposed to a new machine in order to
meet the requirements that:
* the reference hardware be bootable with a h3ulcb machine image to
  avoid expanding the CI test matrix
* the reference hardware support be optional to not hinder Renesas
  BSP maintenance for the h3ulcb / h3ulcb-kf base machines

If the required kernel changes for the reference hardware get
upstreamed and eventually become available via the Renesas BSP this
can be revisited, as the reference hardware support could then likely
be reduced to just building the extra devicetree file(s).

Bug-AGL: SPEC-3658

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Idcf0b8cfe8eb3c5485663ee25519a9c445362829
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25619
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>