net: Handle IPv6 packet reception

When read from the Rx FIFO, pass the network buffer to the uIP IPv6
stack using the tcpip_input() function. As the event is handled, not
all cases where the buffer is to be discarded can be detected.
Thus, if ICMPv6 errors will not be sent, indicate that the packet is
to be dropped by setting its length to zero.

Change-Id: I49f69011685e39759ba25ebb07670308382a5ca8
Signed-off-by: Patrik Flykt <patrik.flykt@linux.intel.com>
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>