For Serial, if assumeSuccess is true, also try to clear the buffers

For Serial, if the Serial port is congested, .begin can fail when the NAV RATE messages are delayed. This is usually a sign that the baud rate is too low. Clearing the serial buffer first seems to help.

src/SparkFun_u-blox_GNSS_Arduino_Library.h

@@ -599,6 +599,7 @@ public:
 	void setPacketCfgPayloadSize(size_t payloadSize); // Set packetCfgPayloadSize
 
 	//Begin communication with the GNSS. Advanced users can assume success if required. Useful if the port is already outputting messages at high navigation rate.
+	//Begin will then return true if "signs of life" have been seen: reception of _any_ valid UBX packet or _any_ valid NMEA header.
 	//By default use the default I2C address, and use Wire port
 	bool begin(TwoWire &wirePort = Wire, uint8_t deviceAddress = 0x42, uint16_t maxWait = defaultMaxWait, bool assumeSuccess = false); //Returns true if module is detected
 	//serialPort needs to be perviously initialized to correct baud rate