homescreen: update recipe

Rework homescreen demo app recipe for building a version of it
with the application framework usage removed.  A simple systemd
unit to start it has been added as a placeholder until the new
application scheme has been added.  This will at least produce
something on screen for testing, and the HOMESCREEN_DEMO_CI
environment variable can be defined via /etc/default/homescreen
to potentially re-enable the CI startup test.

Bug-AGL: SPEC-4121

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I510873d428dc98286775722e23b34cd4a9862258