Background, see:
https://github.com/daniestevez/gr-satellites/issues/208#issuecomment-747060480



Later on we decided to keep the length as well, but to be sure to decode
all packets we handle all threee cases:

  1. No length and HMAC
  2. Length and HMAC
  3. Only HMAC

Only decode as beacon when the csp header says it is a beacon, that is:
  * dstPort = 0x0A (MCC_PORT_BEACON)
  * destination = 0x09 (NODE_MCC)
Signed-off-by: Nick Østergaard <oe.nick@gmail.com>