doc: added description for board revision

This add documentation for the `-DBOARD=<board>@<revision>` feature.

Organized in a user-visible portion in the application development
guide, and implementation details in the board porting guide.

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