net: buf: Introduce support for fragmentation

Introduce support buffer fragment chains that are linked together.
This is done with the help of a flag while the buffer is inside a FIFO
(indicating that the next fragment follows it in the same FIFO) and
with the help of a "next" pointer while the buffer is outside of a
FIFO.

In order to do proper "marshaling" a new net_buf_put() API needs to be
always used when inserting a buffer into a FIFO. Respectively, the
net_buf_get() and net_buf_get_timeout() functions are extended to
support getting buffers from arbitrary FIFOs and reassemble the
fragment chain based on the flags that the received buffers contain.

The insertion of a fragment chain using net_buf_put() into a FIFO is
done atomically with the help of irq_lock/unlock since FIFOs support
multiple writers, however since there's ever only a single reader per
FIFO similar locking is not necessary there.

Change-Id: I0ec579f63ea8d063f50e3f1f4c2e80ec399622d7
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>