samples: mesh_demo: rework error and device handling

The board specific interface in this sample is not handling errors.
Fix that by making board_init() return a negative error code or zero.

Rework the bbc_microbit specific board file by handling errors
appropriately and avoiding device_get_binding() to move some device
configuration errors to build time.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>