ipq806x: nbg6817: add kmod-fs-ext4 to device packages

ipq806x: nbg6817: add kmod-fs-ext4 to device packages

The ZyXEL NBG6817 uses an eMMC flash for the rootfs, which is split
into the readonly squashfs and ext4 for the overlay. This adds the
required package to the device packages to allow mounting the overlay
by default.

/dev/root on /rom type squashfs (ro,relatime)
proc on /proc type proc (rw,nosuid,nodev,noexec,noatime)
sysfs on /sys type sysfs (rw,nosuid,nodev,noexec,noatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
/dev/loop0 on /overlay type ext4 (rw,noatime,data=ordered)
overlayfs:/overlay on / type overlay (rw,noatime,lowerdir=/,upperdir=/overlay/upper,workdir=/overlay/work)
tmpfs on /dev type tmpfs (rw,nosuid,relatime,size=512k,mode=755)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,mode=600,ptmxmode=000)
debugfs on /sys/kernel/debug type debugfs (rw,noatime)
mountd(pid1040) on /tmp/run/blockd type autofs (rw,relatime,fd=7,pgrp=1,timeout=30,minproto=5,maxproto=5,indirect)

Before this commit, the ext4 based overlayfs could not be mounted,
which left only the tmpfs based/ volatile  emergency overlay in place.

Fixes: https://forum.lede-project.org/t/zyxel-nbg6817-flashing-from-oem/768
Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>