Add nRF24 receive message buffering (#538)

* Add critical section support

Allows defining a code block which is executed as a critical section.
Supports AVR, ESP8266 & SAMD.

* Add receive message buffering

When MY_RF24_IRQ_PIN is defined (and connected), a buffer will be used
to retrieve & store messages received by nRF24 as fast as possible.
This reduces the chance of losing messages due to a busy
microcontroller.
nRF24 SPI rx message transfer is running from interrupt context when
MY_RF24_IRQ_PIN is defined. If this transfer takes too long, serial
characters received will get lost. Therefore serial rx is enabled during
SPI transfer of nRF messages.

Limitations: Only AVR/SAMD & nRF24L01+ combinations can buffer rx
messages.
nRF24L01+ IRQ pin should be connected to AVR.
Tested on W5100- & Serial-gateway.

SAMD has not been tested yet!

* Fix Doxygen warnings

* Fix more Doxygen warnings

MySensors.h

@@ -255,6 +255,23 @@
 		#error Only one forward link driver can be activated
 	#endif
 	#if defined(MY_RADIO_NRF24)
+		#ifdef MY_RF24_IRQ_PIN
+			// SoftSPI does not support usingInterrupt()
+			#ifdef MY_SOFTSPI
+				#error RF24 IRQ usage cannot be used with Soft SPI
+			#endif
+			// ESP8266 does not support usingInterrupt()
+			#ifdef ESP8266
+				#error RF24 IRQ usage cannot be used with ESP8266
+			#endif
+			#ifndef SPI_HAS_TRANSACTION
+				#error RF24 IRQ usage requires transactional SPI support
+			#endif
+		#else
+			#ifdef MY_RX_MESSAGE_BUFFER_SIZE
+				#error Receive message buffering requires RF24 IRQ usage
+			#endif
+		#endif
 		#if defined(MY_RF24_ENABLE_ENCRYPTION)
 			#include "drivers/AES/AES.cpp"
 		#endif