Signing support implemented in the MySensors library

The dummy signing driver backend is used. But with this commit all
framework logic for managing signed messages in the MySensors
ecosystem is implemented and enables more sophisticated signing
backends to be added later on.

For a sensor (or gateway) that require a gateway to send signed
messages to it, add 'true' as the fifth argument to 'begin'.
The gateway will automatically inform a node of it's signing
requirements as soon as it receivs the nodes signing preference.
It is important to understand that if the gateway require
signing, it will in this implementation only require signing from
nodes that require signing by the gateway.

If the sensor requires signing from another node on the network
which it communicates directly with, it needs to send 'true' in
an internal message of type I_REQUEST_SIGNING.

Bootloader/MyOtaBootloader.c

@@ -152,6 +152,12 @@ int main () {
 		address(nc.nodeId);
 	}
 	
+	// Inform gateway that bootloader does not accept signed messages
+	msg.type = I_REQUEST_SIGNING;
+	mSetLength(msg, 1);
+	msg.data[0] = 0;
+	sendWrite(msg);
+
 	// Read settings from EEPROM
 	eeprom_read_block((void*)&fc, (void*)EEPROM_FIRMWARE_TYPE_ADDRESS, sizeof(struct FirmwareConfig));