Add template file holding index of variables available to the user

This change add a template file which is appended at the very end so the user sees the available
variables and can modify its settings in a guided way.

Change-Id: I459e853300790e5fce050d6b58d0b07dfd761613
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/10027
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>