Add ability for receiver to check if CRC is present in payload

[mcgurk]

enableCrc() doesn't affect when used with receiver in explicit header mode (datasheet page 30). This modification adds crcOnPayload()-function, which can be used to detect if CRC is present in payload. With this it is possible to verify that CRC is really used between sender and receiver.

SX1276/77/78/79 datasheet page 112: RegHopChannel (0x1C), bit 6: CrcOnPayload (read) CRC Information extracted from the received packet header (Explicit header mode only) 0 = Header indicates CRC off 1 = Header indicates CRC on