AP_Mount: 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_Mount/AP_Mount_Alexmos.cpp

@@ -267,7 +267,7 @@ void AP_Mount_Alexmos::read_incoming()
             case 4: // parsing body
                 _checksum += data;
                 if (_payload_counter < sizeof(_buffer)) {
-                    _buffer.bytes[_payload_counter] = data;
+                    _buffer[_payload_counter] = data;
                 }
                 if (++_payload_counter == _payload_length)
                     _step++;

libraries/AP_Mount/AP_Mount_Alexmos.h

@@ -284,11 +284,11 @@ private:
 
     };
     union PACKED alexmos_parameters {
+        DEFINE_BYTE_ARRAY_METHODS
         alexmos_version version;
         alexmos_angles angles;
         alexmos_params params;
         alexmos_angles_speed angle_speed;
-        uint8_t bytes[0];
     } _buffer,_current_parameters;
 
     AP_HAL::UARTDriver *_port;

libraries/AP_Mount/AP_Mount_SToRM32_serial.cpp

@@ -236,7 +236,7 @@ void AP_Mount_SToRM32_serial::read_incoming() {
             continue;
         }
 
-        _buffer.bytes[_reply_counter++] = data;
+        _buffer[_reply_counter++] = data;
         if (_reply_counter == _reply_length) {
             parse_reply();
 
@@ -266,7 +266,7 @@ void AP_Mount_SToRM32_serial::parse_reply() {
 
     switch (_reply_type) {
         case ReplyType_DATA:
-            crc = crc_calculate(_buffer.bytes, sizeof(_buffer.data)-3);
+            crc = crc_calculate(&_buffer[0], sizeof(_buffer.data) - 3);
             crc_ok = crc == _buffer.data.crc;
             if (!crc_ok) {
                 break;
@@ -277,7 +277,8 @@ void AP_Mount_SToRM32_serial::parse_reply() {
             _current_angle.z = _buffer.data.imu1_yaw;
             break;
         case ReplyType_ACK:
-            crc = crc_calculate(&_buffer.bytes[1], sizeof(SToRM32_reply_ack_struct)-3);
+            crc = crc_calculate(&_buffer[1],
+                                sizeof(SToRM32_reply_ack_struct) - 3);
             crc_ok = crc == _buffer.ack.crc;
             break;
         default:

libraries/AP_Mount/AP_Mount_SToRM32_serial.h

@@ -143,9 +143,9 @@ private:
 
 
     union PACKED SToRM32_reply {
+        DEFINE_BYTE_ARRAY_METHODS
         SToRM32_reply_data_struct data;
         SToRM32_reply_ack_struct ack;
-        uint8_t bytes[0];
     } _buffer;
 
     // keep the last _current_angle values