netifd: Support for configurable default packet steering behavior

netifd: Support for configurable default packet steering behavior

The default packet steering behavior can be configured via the parameter
default_ps in the global section; the default value is true to keep
backwards compatibility.
Device packet steering (rps/xps) config can still be used to override the
default behavior.
This allows you to disable packet steering for all devices without the need
to define a device config list which disables receive/transmit packet steering

Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>