arm-autonomy: Add minimal docker runtime in xenguest images

This patch introduces the 'docker' DISTRO_FEATURE do control whether or not to
have a minimal docker runtime installed in xenguest images.

A new packagegroup and kernel feature were introduced to ensure the minimal
kernel config is applied as well as the packages to be installed in the image.

Change-Id: Id973ca2421c43d5b0978c6f7311e1d0b4db5edca
Issue-Id: SCM-1533
Signed-off-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>