libertas: add sysfs hooks to update boot2 and persistent firmware

To use these features, copy the boot2 and firmware images to /lib/firmware and:

echo <boot2_image_name> > /sys/class/net/ethX/lbs_flash_boot2
echo <firmware_image_name> > /sys/class/net/ethX/lbs_flash_fw

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>