core: frontend: vehicle-setup: allow controlling Sea-level GPS for boats

[ES-Alexander]

Current behaviour

BlueBoat defaults GPS_NAVFILTER to Sea, which works well for standard users but not for those operating in water bodies at elevation (e.g. rivers/lakes on mountains).

As the parameter is set by default, for those users it just seems like the GPS isn't working, and they have no idea why.

Some relevant internal discussion.

Expected or desired behaviour

We should make it possible to configure through the vehicle setup page, with an intuitive explanation about when you would want it on vs off.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.

[patrickelectric]

can't we just have an "AUTO" ?
They appear really simple to be detected...