Copter: flag current alt as relative

This is required so common Log_Write_Camera can calculate relative and
abs altitudes provided by either copter or plane

pair programmed with Craig Elder

ArduCopter/inertia.pde

@@ -12,5 +12,6 @@ static void read_inertial_altitude()
 {
     // with inertial nav we can update the altitude and climb rate at 50hz
     current_loc.alt = inertial_nav.get_altitude();
+    current_loc.flags.relative_alt = true;
     climb_rate = inertial_nav.get_velocity_z();
 }