In some cases (for examples when offloaded socket implementation does
not implement TLS functionality) it could be desired to create a native
TLS socket with an underlying offloaded socket.
This cannot be achieved with SO_BINDTODEVICE option only, as TLS socket
type is not really associated with a particular interface - it either
has to be offloaded, or a fully native socket is created (native TLS on
a native interface).
In order to address the problem, introduce TLS_NATIVE socket option.
This option instructs the socket dispatcher layer to create a native TLS
socket. As with the socket dispatcher the underlying socket
implementation is not decided during TLS socket creation, therefore it's
possible to use SO_BINDTODEVICE to choose either native or offloaded
interface for the underlying socket.
Additionally remove NET_SOCKETS_OFFLOAD_TLS Kconfig option, as it's no
longer needed with an runtime option to select whether to offload TLS or
not.
Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>