Add navigationGetHomeHeading()

Returns the heading for the home point, as recorded when home was
stored. Note that it won't work properly on FW without a mag yet,
since we'll need to adjust the value after we acquire a valid
heading.

src/main/navigation/navigation.c

@@ -2713,6 +2713,12 @@ bool isNavLaunchEnabled(void)
 {
     return IS_RC_MODE_ACTIVE(BOXNAVLAUNCH) || feature(FEATURE_FW_LAUNCH);
 }
+
+int32_t navigationGetHomeHeading(void)
+{
+    return posControl.homePosition.yaw;
+}
+
 #else // NAV
 
 #ifdef USE_GPS