Adding a comment so that readers know what IMU stands for. It appears

there is perhaps some non-IMU code in flight_imu.c and that IMU is too
broad a term to perhaps even name this file after.

src/flight_imu.c

@@ -1,3 +1,5 @@
+// Inertial Measurement Unit (IMU)
+
 #include <stdbool.h>
 #include <stdint.h>
 #include <math.h>