uboot-extlinux-config: exclude OVERRIDES from do_create_extlinux_config vardeps

This function is primarily *appending* configuration entries to the
overrides, it only gets it to ensure it's set at all, so doesn't belong
in the vardeps. Having a dependency on OVERRIDES means you cannot use a
bbclass like distrooverrides without changing the signatures of recipes
using this class.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>