AP_ExternalAHRS: Add pre-arm for misconfigured EAHRS_SENSORS and GPS_TYPE

[Ryanf55]

Purpose

• This catches when there's a mismatch of GPSx_TYPE and EAHRS_SENSORS when GPS is enabled
• Before this pre-arm, failure to set GPS_TYPE2 to 21 (ExternalAHRS) resulted in silent rejection of the data in AP_GPS because the default is off
• The wiki did not say to set GPS_TYPE on anything but the VN300
• For now, until we support 3+ GPS instances, this only checks if you have at least on GPSx_TYPE set to EAHRS.

Drive by fix - logging problem in Microstrain7 with format strings.

Tests

Tested in SITL with MicroStrain7 hardware. And, a new CI test:

./Tools/autotest/autotest.py build.Plane test.Plane.GpsSensorPreArmEAHRS

Why?

The default behavior is to disable the 2nd GPS, so MicroStrain7 was dropping the data on the floor in the call to AP::gps().handle_external(gps, instance);

Futher improvements

The same behavior is possible with other external sensors?