domx: add board config for LOW_SECURE_HEAP

Change-Id: I1a888975945ec88d011e3bfc4e8af7d8ac36fc59
camera: bugfixes for updated camera HAL

- Add MAX_CAMERAS board config to limit scanning of omx cameras
(Jem has 2 extra configs which don't exist)
- Add TREAT_FRONT_AS_BACK board config to pass the front camera
off to the framework as a back camera to fix Panorama modes
- ifdef VT slices code which is only used in CPCAM mode

Change-Id: I273ec01403e216c544722e681c96ae7e1220b8d7