net/pkt: Introducing a new helper to get the start of the buffer

This is mostly meant to be used in drivers and/or L2.

net_pkt_ll() is going to be removed and is not semantically right.
Also, net_pkt_ip_data() is not semantically right as it is meant to
access IP data start.

So instead, adding a new function: net_pkt_data() which will get the
start of the buffer.

Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>