Introduce a new interface state management scheme, according to
RFC 2863. This includes the following changes:
* Introduce a new interface flag: NET_IF_LOWER_UP, along with
corresponding helper functions. The flag should be set/cleared on an
interface by a network driver/L2 to signalize physical readiness of an
interface to transmit data (for example cable plugged in).
* Introduce a new interface flag: NET_IF_DORMANT, along with
corresponding helper functions. The flag should be set on an
iterface when the interface is not ready to transmit application data,
for example still not joined a Wi-Fi network.
* Introduce a new interface flag: NET_IF_RUNNING, indicating that
interface is ready to transmit application data.
* Update the meaning of the NET_IF_UP flag - it now singnalizes whether
an interface has been brought up/down by the application (admin
up/down).
* Introduce operational state of an interface, derived from above. It
reflects the internal interface state.
The meaning of net_if_is_up() function and NET_EVENT_IF_UP/DOWN events
remains unchanged to retain backward compability - they reflect the
interface readiness to transmit application data.
To verify the administrative up/down state, a new function
net_if_is_admin_up() has been introduced, along with
NET_EVENT_IF_ADMIN_UP/DOWN events.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
efd03a86d7