Implement new parameter for GNSS system selection

[jbeyerstedt]

A new PX4 parameter is needed to configure the set of used GNSS systems. This PR changes the GPSHelper::configure() interface method used by the GPS drivers to include another parameter for the configuration (with the same data format, as the PX4 parameter).
Additionally the actual feature of setting the GNSS configuration was implemented for uBlox devices.

See #68 for details.

[jbeyerstedt]

I decided to not add a default value to the new parameter of that method, because it isn't needed any more when the corresponding change in PX4 was merged. But therefore the changes in this repository and in PX4 depend on each other. Let me know, if this is an issue.
My proposal for the procedure would be to merge this first and do the submodule update in PX4 as part of the PX4/PX4-Autopilot#16505 pull request.

[jbeyerstedt]

And could somebody please test this with a uBlox generation 9 receiver (or some other one, which is using at least protocol version 27/ the config-valset interface)? I only have a Pixhawk 4 GPS, which is using the configureDevicePreV27() code.

[bkueng]

Thanks, I'll test this on a newer module.

[bkueng]

I tested with an F9P, but it does not work, I always get a NACK: setting UBX_CFG_KEY_SIGNAL_SBAS_ENA consistently fails, either enabling or disabling. The same result with u-center, so I think the code is correct, and I don't know why it fails. We could apply SBAS separately so at least the other settings apply.

I also noticed a bug: https://github.com/PX4/PX4-GPSDrivers/blob/master/src/ubx.cpp#L1294 should use ubx_sat_svId.

[jbeyerstedt]

The SBAS settings have been moved to the bottom of the section, committing the other changes beforehand. Additionally I explicitly set the used signal for SBAS now (even tough it should be set to that value by default). Maybe this works better.

The other bug is addressed im #70.