AP_GPS: use ARRAY_SUBSCRIPT instead of bytes field

That fixed compilation issues and seems more semantically correct. Using array
of length 0 fails compilation because of -Werror=array-bounds in GCC 6.1.

libraries/AP_GPS/AP_GPS_ERB.cpp

@@ -110,7 +110,7 @@ AP_GPS_ERB::read(void)
         case 5:
             _ck_b += (_ck_a += data);                   // checksum byte
             if (_payload_counter < sizeof(_buffer)) {
-                _buffer.bytes[_payload_counter] = data;
+                _buffer[_payload_counter] = data;
             }
             if (++_payload_counter == _payload_length)
                 _step++;

libraries/AP_GPS/AP_GPS_ERB.h

@@ -85,12 +85,12 @@ private:
 
     // Receive buffer
     union PACKED {
+        DEFINE_BYTE_ARRAY_METHODS
         erb_ver ver;
         erb_pos pos;
         erb_stat stat;
         erb_dops dops;
         erb_vel vel;
-        uint8_t bytes[0];
     } _buffer;
 
     enum erb_protocol_bytes {

libraries/AP_GPS/AP_GPS_MTK.cpp

@@ -115,7 +115,7 @@ restart:
         // Receive message data
         //
         case 4:
-            _buffer.bytes[_payload_counter++] = data;
+            _buffer[_payload_counter++] = data;
             _ck_b += (_ck_a += data);
             if (_payload_counter == sizeof(_buffer))
                 _step++;

libraries/AP_GPS/AP_GPS_MTK.h

@@ -71,8 +71,8 @@ private:
 
     // Receive buffer
     union PACKED {
+        DEFINE_BYTE_ARRAY_METHODS
         diyd_mtk_msg msg;
-        uint8_t bytes[0];
     } _buffer;
 
     // Buffer parse & GPS state update

libraries/AP_GPS/AP_GPS_MTK19.cpp

@@ -105,7 +105,7 @@ restart:
         // Receive message data
         //
         case 3:
-            _buffer.bytes[_payload_counter++] = data;
+            _buffer[_payload_counter++] = data;
             _ck_b += (_ck_a += data);
             if (_payload_counter == sizeof(_buffer)) {
                 _step++;

libraries/AP_GPS/AP_GPS_MTK19.h

@@ -77,7 +77,7 @@ private:
 
     // Receive buffer
     union {
+        DEFINE_BYTE_ARRAY_METHODS
         diyd_mtk_msg msg;
-        uint8_t bytes[0];
     } _buffer;
 };

libraries/AP_GPS/AP_GPS_SIRF.cpp

@@ -138,7 +138,7 @@ AP_GPS_SIRF::read(void)
         case 5:
             if (_gather) {                                              // gather data if requested
                 _accumulate(data);
-                _buffer.bytes[_payload_counter] = data;
+                _buffer[_payload_counter] = data;
                 if (++_payload_counter == _payload_length)
                     _step++;
             } else {

libraries/AP_GPS/AP_GPS_SIRF.h

@@ -97,8 +97,8 @@ private:
 
     // Message buffer
     union {
+        DEFINE_BYTE_ARRAY_METHODS
         sirf_geonav nav;
-        uint8_t bytes[0];
     } _buffer;
 
     bool        _parse_gps(void);

libraries/AP_GPS/AP_GPS_UBLOX.cpp

@@ -431,7 +431,7 @@ AP_GPS_UBLOX::read(void)
         case 6:
             _ck_b += (_ck_a += data);                   // checksum byte
             if (_payload_counter < sizeof(_buffer)) {
-                _buffer.bytes[_payload_counter] = data;
+                _buffer[_payload_counter] = data;
             }
             if (++_payload_counter == _payload_length)
                 _step++;

libraries/AP_GPS/AP_GPS_UBLOX.h

@@ -360,6 +360,7 @@ private:
 
     // Receive buffer
     union PACKED {
+        DEFINE_BYTE_ARRAY_METHODS
         ubx_nav_posllh posllh;
         ubx_nav_status status;
         ubx_nav_dop dop;
@@ -384,7 +385,6 @@ private:
         ubx_rxm_rawx rxm_rawx;
 #endif
         ubx_ack_ack ack;
-        uint8_t bytes[0];
     } _buffer;
 
     enum ubs_protocol_bytes {