The packages necessary to get the DSL modem working increase the
squashfs size by around 1MB.

Remove them from Gluon, as this functionality is not supported.

(cherry picked from commit 43b95cfc)

There is no factory image, they are flashed using the sysupgrade image.

Flash instructions in the OpenWRT Commit: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=eae6cac6a30b95bef04d033ed8a303b7e35a078b