Let's do all the submodule stuff in check_submodules.sh

Previously make would override a submodule, now it only does submodule
init and override if the submodule is not already checked out.

This enables continuous mode for the HMC5883 instead of single
measurement mode, therefore working around a ADSP crash.

This code is flyable, but a few problems exist which cause values in the .config files to be overwritten by the defaults in the .XML file.

Original description:
Modified the sensor module to prevent the selection of an invalid
secondary/tertiary gyro if the primary gyro times out