buildroot

configs/imx6ulpico: Bump to kernel 4.8

Now that the final kernel 4.8 has been released, let's use it
instead of 4.8-rc5.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>