realtek: merge Zyxel GS1900 firmware partitions

realtek: merge Zyxel GS1900 firmware partitions

The dual-boot partition layout for the Zyxel GS1900 switches results in
6.9MB for both kernel and rootfs. Depending on the package selection,
this may already leave no space for the user overlay.

Merge the two firmware partitions, effectively dropping dual boot
support with OpenWrt. This results in a firmware partition of 13.9MB,
which should leave some room for the future.

To maintain install capabilites on new devices, an image is required
that still fits inside the original partition. The initramfs is used as
factory install image, so ensure this meets the old size constraints.
The factory image can be flashed via the same procedure as vendor images
when reverting to stock, can be installed from stock, or can be launched
via tftpboot.

Link: https://github.com/openwrt/openwrt/issues/16439
Link: https://github.com/openwrt/openwrt/pull/16442
Tested-by: Stijn Segers <foss@volatilesystems.org>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
(cherry picked from commit 35acdbe9095d81e896a2dfa65e7df871a023b996)