lxc-host-image-minimal: Handle Kingfisher image names

lxc-host-image-minimal: Handle Kingfisher image names

To handle the Kingfisher supporting ULCB builds modifying the
IMAGE_LINK_NAME variable, add a MACHINE_SUFFIX variable that is
defined based on a new "kingfisher" feature being present in
AGL_FEATURES.  Since so far only one platform has this issue,
this simple approach seems better than attempting to derive
the suffix from IMAGE_LINK_NAME using anonymous Python.

Bug-AGL: SPEC-4023

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ida769fbeb57208934f738dc5d1c867ab63c40bff