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/MyHwESP8266.h

@@ -43,5 +43,17 @@ void hwWriteConfigBlock(void* buf, void* adr, size_t length);
 void hwWriteConfig(int adr, uint8_t value);
 uint8_t hwReadConfig(int adr);
 
+/**
+ * Restore interrupt state.
+ * Helper function for MY_CRITICAL_SECTION.
+ */
+static __inline__ void __psRestore(const uint32_t *__s)
+{
+    xt_wsr_ps( *__s );
+}
+   
+#ifndef DOXYGEN
+	#define MY_CRITICAL_SECTION    for ( uint32_t __psSaved __attribute__((__cleanup__(__psRestore))) = xt_rsil(15), __ToDo = 1; __ToDo ; __ToDo = 0 )
+#endif  /* DOXYGEN */
 
 #endif // #ifdef ARDUINO_ARCH_ESP8266