usertools: use optimized driver override scheme to bind

usertools: use optimized driver override scheme to bind

The current device bind model uses /sys/bus/pci/driver/new_id scheme to
bind devices to the driver. This scheme has following operations to bind
a device to the driver.
1) Write device ID and vendor ID to /sys/bus/pci/driver/new_id
2) Write PCI BDF number to /sys/bus/pci/driver/bind
3) On step (1), _All_ the devices that match the device ID and vendor ID
get bound to the driver
4) Except for requested devices, Unbind the remaining devices

In kernels >= 3.15, An alternative scheme driver_override can be used to
bind a device to driver.This scheme has following operations to bind a
device to driver.
1) Write driver to /sys/bus/pci/device/driver_override
2) Write PCI BDF number to /sys/bus/pci/driver/bind

This script detects the presence of /sys/bus/pci/device/driver_override,
if available use optimized bind scheme to bind it

Signed-off-by: Guduri Prathyusha <gprathyusha@caviumnetworks.com>
Signed-off-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>