Allow User select Tablet Mode

Device overlay variables for tablet mode on Tuna.

Needs other Tablet UI commits.

Change-Id: I5529c884b54e2907ec762ee00d6cb94149d6b7df
Signed-off-by: Zaphod <mwilson@acesappraisal.com>
Signed-off-by: DHO <DHO@dho.im>