hmi-controller: allocate base layers for multi-screen

A layer ID for screen is set by key: base-layer-id at weston.ini. To
support multi screens. It also support offset to offset the layer ID
to next ID for next screen.

For example,
base-layer-id=1000
base-layer-id-offset=10000

Layer id for screen 0: 1000
Layer id for screen 1: 11000
Layer id for screen 2: 21000

To support multi screen, create layers for background and panel bar which
located in the below per screens. At the moment, it is only layers.
Surfaces to be created at next patch in the patch set.

Signed-off-by: Nobuhiko Tanibata <NOBUHIKO_TANIBATA@xddp.denso.co.jp>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>