Enable buffered writes/transmissions

- Add optional startTx to startFastWrite() function:
a: Allows software to fully utilize FIFO buffers for buffering payloads
to the TX FIFO, while remaining in RX mode, instead of storing data in
RAM etc.
b: Default is still to start sending immediately unless startTx is
included and set to 0

Note: This will not work correctly if using ack-payloads. Tx FIFO is
flushed when switching between RX/TX modes to prevent data corruption.

- Added optional startTx to txStandBy(timeout,startTx)
a: Allows automated sending of buffered data, with indication of success
or failure for the bulk data transfer.
b: Default is still to not initiate sending, and not leave RX mode.
c: Calling this with startTx set to true will automatically call
radio.stopListening() to begin transmission.

Example:
radio.startFastWrite( &time, sizeof(unsigned long), false );
radio.startFastWrite( &time, sizeof(unsigned long), false );

if (!radio.txStandBy(100, true) ){  printf("failed.\n\r");  }
radio.startListening()

RF24.cpp

@@ -958,11 +958,13 @@ bool RF24::writeFast( const void* buf, uint8_t len ){
 //Otherwise we enter Standby-II mode, which is still faster than standby mode
 //Also, we remove the need to keep writing the config register over and over and delaying for 150 us each time if sending a stream of data
 
-void RF24::startFastWrite( const void* buf, uint8_t len, const bool multicast){ //TMRh20
+void RF24::startFastWrite( const void* buf, uint8_t len, const bool multicast, bool startTx){ //TMRh20
 
 	//write_payload( buf,len);
 	write_payload( buf, len,multicast ? W_TX_PAYLOAD_NO_ACK : W_TX_PAYLOAD ) ;
-	ce(HIGH);
+	if(startTx){
+		ce(HIGH);
+	}
 
 }
 
@@ -993,6 +995,7 @@ bool RF24::rxFifoFull(){
 /****************************************************************************/
 
 bool RF24::txStandBy(){
+
     #if defined (FAILURE_HANDLING) || defined (RF24_LINUX)
 		uint32_t timeout = millis();
 	#endif
@@ -1019,8 +1022,12 @@ bool RF24::txStandBy(){
 
 /****************************************************************************/
 
-bool RF24::txStandBy(uint32_t timeout){
+bool RF24::txStandBy(uint32_t timeout, bool startTx){
 
+    if(startTx){
+	  stopListening();
+	  ce(HIGH);
+	}
 	uint32_t start = millis();
 
 	while( ! (read_register(FIFO_STATUS) & _BV(TX_EMPTY)) ){
@@ -1047,6 +1054,7 @@ bool RF24::txStandBy(uint32_t timeout){
 	return 1;
 
 }
+
 /****************************************************************************/
 
 void RF24::maskIRQ(bool tx, bool fail, bool rx){

RF24.h

@@ -465,7 +465,7 @@ s   *
    * @return True if transmission is successful
    *
    */
-   bool txStandBy(uint32_t timeout);
+   bool txStandBy(uint32_t timeout, bool startTx = 0);
 
   /**
    * Write an ack payload for the specified pipe
@@ -545,7 +545,7 @@ s   *
    * @param multicast Request ACK (0) or NOACK (1)
    * @return True if the payload was delivered successfully false if not
    */
-  void startFastWrite( const void* buf, uint8_t len, const bool multicast );
+  void startFastWrite( const void* buf, uint8_t len, const bool multicast, bool startTx = 1 );
 
   /**
    * Non-blocking write to the open writing pipe
@@ -568,7 +568,7 @@ s   *
    *
    */
   void startWrite( const void* buf, uint8_t len, const bool multicast );
-
+  
   /**
    * This function is mainly used internally to take advantage of the auto payload
    * re-use functionality of the chip, but can be beneficial to users as well.