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

core/MyHwATMega328.h

@@ -27,7 +27,7 @@
 #include <avr/power.h>
 #include <avr/interrupt.h>
 #include <avr/wdt.h>
-
+#include <util/atomic.h>
 
 
 #ifdef __cplusplus
@@ -92,4 +92,8 @@ enum period_t
 
 void hwInternalSleep(unsigned long ms);
 
+#ifndef DOXYGEN
+  #define MY_CRITICAL_SECTION     ATOMIC_BLOCK(ATOMIC_RESTORESTATE)
+#endif  /* DOXYGEN */
+
 #endif