GCS_MAVLink: take 0,0 as meaning set-home-to-current-location

This isn't in spec, but is what Sub used to do, and what other vehicles
also used to do.

We treat 0,0 as "current location" in various other places, so it kind
of makes sense here too.

libraries/GCS_MAVLink/GCS_Common.cpp

@@ -3523,8 +3523,8 @@ MAV_RESULT GCS_MAVLINK::handle_command_mount(const mavlink_command_long_t &packe
 
 MAV_RESULT GCS_MAVLINK::handle_command_do_set_home(const mavlink_command_long_t &packet)
 {
-    // if param1 is 1 use current location:
+    if (is_equal(packet.param1, 1.0f) || (is_zero(packet.param5) && is_zero(packet.param6))) {
-    if (is_equal(packet.param1, 1.0f)) {
+        // param1 is 1 (or both lat and lon are zero); use current location
         if (!set_home_to_current_location(true)) {
             return MAV_RESULT_FAILED;
         }
@@ -3707,8 +3707,8 @@ MAV_RESULT GCS_MAVLINK::handle_command_do_set_roi(const Location &roi_loc)
 
 MAV_RESULT GCS_MAVLINK::handle_command_int_do_set_home(const mavlink_command_int_t &packet)
 {
-    if (is_equal(packet.param1, 1.0f)) {
+    if (is_equal(packet.param1, 1.0f) || (packet.x == 0 && packet.y == 0)) {
-        // if param1 is 1, use current location
+        // param1 is 1 (or both lat and lon are zero); use current location
         if (!set_home_to_current_location(true)) {
             return MAV_RESULT_FAILED;
         }