Raspberry Pi support

Added support for hardware SPI on RPI per jscrane fork:
- Fixed a few bugs with single SPI transfers
- Aligned with optimized RF24 library fork
- Updated bmc2835 files per
http://www.airspayce.com/mikem/bcm2835/index.html
- Merged bcm files with changes from
https://github.com/farconada/RF24Network
- Added example files
- Added setup info to readme

RPi/RF24/examples/gettingstarted_call_response.cpp

@@ -0,0 +1,138 @@
+/*
+TMRh20 2014 - Updated to work with optimized RF24 Arduino library
+*/
+
+
+/**
+ * Example for efficient call-response using ack-payloads
+ *
+ * This example continues to make use of all the normal functionality of the radios including
+ * the auto-ack and auto-retry features, but allows ack-payloads to be written optionlly as well.
+ * This allows very fast call-response communication, with the responding radio never having to
+ * switch out of Primary Receiver mode to send back a payload, but having the option to switch to
+ * primary transmitter if wanting to initiate communication instead of respond to a commmunication.
+ */
+
+#include <cstdlib>
+#include <iostream>
+#include <sstream>
+#include <string>
+#include "./RF24.h"
+
+using namespace std;
+
+//
+// Hardware configuration
+//
+
+// CE Pin, CSN Pin, SPI Speed
+
+// Setup for GPIO 22 CE and GPIO 25 CSN with SPI Speed @ 1Mhz
+//RF24 radio(RPI_V2_GPIO_P1_22, RPI_V2_GPIO_P1_18, BCM2835_SPI_SPEED_1MHZ);
+
+// Setup for GPIO 22 CE and CE0 CSN with SPI Speed @ 4Mhz
+//RF24 radio(RPI_V2_GPIO_P1_15, BCM2835_SPI_CS0, BCM2835_SPI_SPEED_4MHZ);
+
+// Setup for GPIO 22 CE and CE1 CSN with SPI Speed @ 8Mhz
+RF24 radio(RPI_V2_GPIO_P1_15, BCM2835_SPI_CS0, BCM2835_SPI_SPEED_8MHZ);
+
+
+
+// Radio pipe addresses for the 2 nodes to communicate.
+const uint64_t addresses[2] = { 0xABCDABCD71LL, 0x544d52687CLL };
+
+bool role_ping_out = 1, role_pong_back = 0, role = 0;
+
+uint8_t counter = 1;                                                          // A single byte to keep track of the data being sent back and forth
+
+
+int main(int argc, char** argv){
+
+
+  printf("RF24/examples/gettingstarted_call_response\n");
+
+  radio.begin();
+  radio.setAutoAck(1);                    // Ensure autoACK is enabled
+  radio.enableAckPayload();               // Allow optional ack payloads
+  radio.setRetries(1,15);                 // Smallest time between retries, max no. of retries
+  radio.setPayloadSize(1);                // Here we are sending 1-byte payloads to test the call-response speed
+  //radio.powerUp();
+  radio.printDetails();                   // Dump the configuration of the rf unit for debugging
+
+
+/********* Role chooser ***********/
+
+  printf("\n ************ Role Setup ***********\n");
+  string input = "";
+  char myChar = {0};
+  cout << "Choose a role: Enter 0 for pong_back, 1 for ping_out (CTRL+C to exit)\n>";
+  getline(cin,input);
+
+  if(input.length() == 1) {
+	myChar = input[0];
+	if(myChar == '0'){
+		cout << "Role: Pong Back, awaiting transmission " << endl << endl;
+	}else{  cout << "Role: Ping Out, starting transmission " << endl << endl;
+		role = role_ping_out;
+	}
+  }
+/***********************************/
+  // This simple sketch opens two pipes for these two nodes to communicate
+  // back and forth.
+
+    if ( role == role_ping_out )    {
+      radio.openWritingPipe(addresses[0]);
+      radio.openReadingPipe(1,addresses[1]);
+    }else{
+      radio.openWritingPipe(addresses[1]);
+      radio.openReadingPipe(1,addresses[0]);
+      radio.startListening();
+    }
+
+// forever loop
+while (1){
+
+
+/****************** Ping Out Role ***************************/
+
+  if (role == role_ping_out){                               // Radio is in ping mode
+
+    uint8_t gotByte;                                           // Initialize a variable for the incoming response
+
+    radio.stopListening();                                  // First, stop listening so we can talk.
+    printf("Now sending %d as payload. ",counter);          // Use a simple byte counter as payload
+    unsigned long time = millis();                          // Record the current microsecond count
+
+    if ( radio.write(&counter,1) ){                         // Send the counter variable to the other radio
+        if(!radio.available()){                             // If nothing in the buffer, we got an ack but it is blank
+            printf("Got blank response. round-trip delay: %lu ms\n\r",millis()-time);
+        }else{
+            while(radio.available() ){                      // If an ack with payload was received
+                radio.read( &gotByte, 1 );                  // Read it, and display the response time
+                printf("Got response %d, round-trip delay: %lu ms\n\r",gotByte,millis()-time);
+                counter++;                                  // Increment the counter variable
+            }
+        }
+
+    }else{        printf("Sending failed.\n\r"); }          // If no ack response, sending failed
+
+    sleep(1);  // Try again later
+  }
+
+/****************** Pong Back Role ***************************/
+
+  if ( role == role_pong_back ) {
+    uint8_t pipeNo, gotByte;                       // Declare variables for the pipe and the byte received
+    while( radio.available(&pipeNo)){              // Read all available payloads
+      radio.read( &gotByte, 1 );
+                                                   // Since this is a call-response. Respond directly with an ack payload.
+      		  				                       // Ack payloads are much more efficient than switching to transmit mode to respond to a call
+      radio.writeAckPayload(pipeNo,&gotByte, 1 );  // This can be commented out to send empty payloads.
+      printf("Sent response %d \n\r", gotByte);
+   }
+ }
+
+} //while 1
+} //main
+
+