net: Switch from per-iface tx thread to a unique k_poll triggered one

Now that k_poll landed in the kernel, it's worth using it to save
memory and reduce the number of threads at runtime.

Such switch has been first done in bluetooth (see hci_core.c and conn.c
in subsys/bluetooth/host). Since network interfaces kind of follows the
same design for sending data, it was then easy to copy the same change as
in bluetooth.

Change-Id: I7f9734b88ac818284bbabaedc946b4765b905ebb
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>