net: pkt: Add possibility to reserve link layer header

Allow network device driver to configure the system so that
when a network packet is sent to it, the link layer header
is stored just before the L2 payload in the same packet.

Currently the link layer header is stored in a separate
net_buf that is linked in front of the L2 payload.

This option can typically save one net_buf when sending
a network packet. Note that if you are using variable data
size buffers (CONFIG_NET_BUF_VARIABLE_DATA_SIZE) then this
embedding is not needed because one can allocate just the
right size network buffers and not waste any memory.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>