core: frontend: bridges: support lower/custom baud rates

[ES-Alexander]

Current behaviour

The minimum baudrate is currently 9600, which is too fast for some devices to be able to operate.

Expected or desired behaviour

We should support slower baud rates, either explicitly by extending the main set of available ones, or via a "custom" option that allows specifying an arbitrary integer rate (which would avoid needing to check a bunch of extra rates, and could allow greater versatility, at the expense of some code and interface complexity).

This may be resolved with #1093, if the Serial Bridges and MAVLink Endpoints pages got merged into a more comprehensive "linking" page (perhaps as separate tabs within it).

Prerequisites

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