kconfigs: hikey_debian: Set CONFIG_EXT4_FS=y

Follow reference in
https://github.com/96boards/meta-96boards/blob/master/recipes-kernel/linux/linux-hikey_git.bb



do_config() {
	..
	echo 'CONFIG_EXT4_FS=y' >> ${B}/.config
	..
}

Signed-off-by: Victor Chong <victor.chong@linaro.org>
Acked-by: Jerome Forissier <jerome.forissier@linaro.org>