Having a generic interface over the GPS drivers makes dedicated
functionality for each driver harder. Move the Septentrio driver into
its own module under the `gnss` driver directory, and let it have its
own parameters for only the functionality it requires. This also helps
with adding new features because they only need to be implemented for
the driver that wants it, simplifying testing.

Add support for two receivers which was also present in the original GPS
drivers. Also fix the reset command which didn't work.

This removes the SBF code from the common GPS drivers which frees up
space to include the Septentrio driver module by default on some boards.

Add back parameters to choose baud rate and automatically detect baud
rate to use in configuration.

Rewrite large parts from the original driver to make the code more
understandable, maintainable, document more items, move related things
into their own files and make the code more consistent.