Adding RTE_KNI_PREEMPT configuration option

This patch introduces CONFIG_RTE_KNI_PREEMPT flag. When set to 'no', KNI
kernel thread(s) do not call schedule_timeout_interruptible(), which improves
overall KNI performance at the expense of CPU cycles (polling).

Default values is 'yes', maintaining the same behaviour as of now.

Signed-off-by: Marc Sune <marc.sune@bisdn.de>