Move the network buffer header file from zephyr/net/buf.h to
zephyr/net_buf.h as the implementation now lives outside of the networking
subsystem.
Add (deprecated) zephyr/net/buf.h header to maintain compatibility with old
file path.
Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>
Move the net_buf implementation from the networking subsystem to a library
as they have no dependency on the networking subsystem.
Network buffers are used in subsystems outside of networking
(e.g. Bluetooth, USB).
Fixes: #36374
Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>