armv8r64-extras/kas: Expose virtualization config as env vars

To make it easier to make simple changes to the virtualization
stack, expose the following environment variables through the Kas
configuration:
 * XEN_DOM0LESS_DOMAINS, so the list of domains can be configured.
 * XEN_DOM0LESS_LINUX_IMAGE, so the included Linux image can be
   configured.
 * XEN_DOM0LESS_ZEPHYR_APPLICATION, so the included Zephyr application
   can be configured.

Issue-Id: SCM-4387
Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I11a229963bb1fc37591d11b6a85b904f43a4a0e0