net/failsafe: add flexible device definition

net/failsafe: add flexible device definition

Add the "exec" device type.
The parameters given to this type of device will be executed in a shell.
The output of this command is then used as a definition for a device.

That command can be re-interpreted if the related device is not
plugged-in. It allows for a device definition to react to system
changes (e.g. changing PCI bus for a given device).

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>
Acked-by: Olga Shern <olgas@mellanox.com>