Enhance randomness (AVR, ESP8266, Linux, NRF5) (#845)

This commit changes the way random numbers are available
with MySensors.

The AVR platform uses a 32 Bit random seed build from a floating
analog pin. The way of the generation of this seed is FIPS 140-2
compliant.

For support MCU with a fast way to generate random data, the
hwGetentropy() command is now available to allow rapid generation
of random data.

The ESP8266 platform uses the internal random source for randomness
for hwGetentropy() and Arduino's random() function.

The NRF5 platform initializes the pseudo random number generator
from internal hardware number generator. The internal AES-ECB unit
generates random data after filling with HWRNG data by AES-CBC.

On Linux getrandom() is implemented via syscall().

signerAtsha204SoftGetNonce speed:
AVR = ~14000us (sha204Soft)
NRF51 = ~152us (AES-CBC), 1739us (old sha204Soft)
NRF52 = ~31us (AES-CBC), 183us (old sha204Soft)
ESP8266 = ~20us (RANDOM_REG32), 380us (old sha204Soft)

All Nonce results tested for FIPS 140-2 compliance successfully.

core/MySigningAtsha204Soft.cpp

@@ -109,6 +109,10 @@ bool signerAtsha204SoftGetNonce(MyMessage &msg)
 	}
 	SIGN_DEBUG(PSTR("Signing backend: ATSHA204Soft\n"));
 
+#ifdef MY_HW_HAS_GETRANDOM
+	// Try to get MAX_PAYLOAD random bytes
+	while (hwGetentropy(&_signing_verifying_nonce, MAX_PAYLOAD) != MAX_PAYLOAD);
+#else
 	// We used a basic whitening technique that XORs a random byte with the current hwMillis() counter and then the byte is
 	// hashed (SHA256) to produce the resulting nonce
 	_signing_sha256.init();
@@ -116,6 +120,7 @@ bool signerAtsha204SoftGetNonce(MyMessage &msg)
 		_signing_sha256.write(random(256) ^ (hwMillis()&0xFF));
 	}
 	memcpy(_signing_verifying_nonce, _signing_sha256.result(), MAX_PAYLOAD);
+#endif
 
 	// We set the part of the 32-byte nonce that does not fit into a message to 0xAA
 	memset(&_signing_verifying_nonce[MAX_PAYLOAD], 0xAA, sizeof(_signing_verifying_nonce)-MAX_PAYLOAD);