Copter: Split out prearm failures of low HDOP separately from !gps lock

(on the iris list there was some confusion on why the copter was blinking
yellow but yet they had gps lock)

ArduCopter/motors.pde

@@ -410,13 +410,21 @@ static bool pre_arm_gps_checks(bool display_failure)
     float speed_cms = inertial_nav.get_velocity().length();     // speed according to inertial nav in cm/s
 
     // ensure GPS is ok and our speed is below 50cm/s
-    if (!GPS_ok() || g_gps->hdop > g.gps_hdop_good || gps_glitch.glitching() || speed_cms == 0 || speed_cms > PREARM_MAX_VELOCITY_CMS) {
+    if (!GPS_ok() || gps_glitch.glitching() || speed_cms == 0 || speed_cms > PREARM_MAX_VELOCITY_CMS) {
         if (display_failure) {
             gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: Bad GPS Pos"));
         }
         return false;
     }
 
+    // warn about hdop separately - to prevent user confusion with no gps lock
+    if (g_gps->hdop > g.gps_hdop_good) {
+        if (display_failure) {
+            gcs_send_text_P(SEVERITY_HIGH,PSTR("PreArm: High GPS HDOP"));
+        }
+        return false;
+    }
+
     // if we got here all must be ok
     return true;
 }