net: Allow user application to get the packet data length

The uip_len(buf) cannot be used as the Contiki stack zeroes
the buffer len after sending and after processing received packet.

Thus the application can use uip_appdatalen(buf) macro to get
the application data length.

Change-Id: Iaaa508acb96c28919cfc0188ef46bb2b4a693090
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>