HAL_AVR: tweak the default serial buffer sizes

this reduces memory fragmentation a lot, saving memory

libraries/AP_HAL_AVR/HAL_AVR_APM2_Class.cpp

@@ -58,7 +58,7 @@ void HAL_AVR_APM2::init(int argc, char * const argv[]) const {
    
     /* uartA is the serial port used for the console, so lets make sure
      * it is initialized at boot */
-    uartA->begin(115200);
+    uartA->begin(115200, 128, 128);
     console->init((void*)uartA);
     /* The AVR RCInput drivers take an AP_HAL_AVR::ISRRegistry*
      * as the init argument */

libraries/AP_HAL_AVR/UARTDriver.h

@@ -110,15 +110,12 @@ private:
 	static void _freeBuffer(Buffer *buffer);
 
 	/// default receive buffer size
-	static const uint16_t _default_rx_buffer_size = 128;
+	static const uint16_t _default_rx_buffer_size = 4;
 
 	/// default transmit buffer size
 	static const uint16_t _default_tx_buffer_size = 16;
 
 	/// maxium tx/rx buffer size
-	/// @note if we could bring the max size down to 256, the mask and head/tail
-	///       pointers in the buffer could become uint8_t.
-	///
 	static const uint16_t _max_buffer_size = 256;
 };