Add an on-target microbenchmark for LatLonAlt::Wgs84::gravity(), the
Somigliana latitude-dependent gravity used by EKF2 (APX4-4273), to
bound its CPU cost on the flight controller.
Measured ~0.45 us/call on fmu-v6s (STM32H7). The value is cached and
only recomputed when latitude moves >1 deg, so the steady-state EKF
cost is unchanged.
Enabled on the fmu-v6s performance-test board. Run with:
nsh> microbench microbench_gravity
APX4-4273