net: if: Add NET_IF_UP flag

This adds NET_IF_UP flag support indicating the interface is up,
currently this shall only be used internally by the driver, later on it
shall be possible to make it public by using dedicated functions.

Change-Id: I38090da4030395b2341733b846004789416d61c1
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>