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

MyConfig.h

@@ -429,6 +429,22 @@
 	#endif
 #endif
 
+/**
+ * @def MY_RF24_IRQ_PIN
+ * @brief Disable RF24 interrupt pin usage by default. Override in sketch if needed.
+ */
+//#define MY_RF24_IRQ_PIN
+
+/**
+ * @def MY_RX_MESSAGE_BUFFER_SIZE
+ * @brief Declare the amount of incoming messages that can be buffered. Requires MY_RADIO_NRF24 and MY_RF24_IRQ_PIN. Override in sketch if needed.
+ */
+#ifdef MY_RF24_IRQ_PIN
+	#ifndef MY_RX_MESSAGE_BUFFER_SIZE
+		#define MY_RX_MESSAGE_BUFFER_SIZE  (20)
+	#endif
+#endif
+
 /**
  * @def MY_RF24_PA_LEVEL
  * @brief Default RF24 PA level. Override in sketch if needed.
@@ -694,5 +710,7 @@
 #define MY_REGISTRATION_CONTROLLER
 #define MY_DEBUG_VERBOSE_RF24
 #define MY_TRANSPORT_SANITY_CHECK
+#define MY_RF24_IRQ_PIN
+#define MY_RX_MESSAGE_BUFFER_SIZE
 #define MY_NODE_LOCK_FEATURE
 #endif