greybus: makefile: add needed config options for lights

Add a function to check kernel versions and append the necessary
options to support LEDS_CLASS, LEDS_CLASS_FLASH and  V4L2_FLASH_LED_CLASS
depending of the kernel version.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>