uboot-envtools: add support for ZyXEL GS-1900-24EP

uboot-envtools: add support for ZyXEL GS-1900-24EP

It seems the that this was forgotten during initial adding of the
device in 0688cf5aebe1dc9a2e7f3820861783c2a7a75d44

Thanks to
https://forum.openwrt.org/t/zyxel-gs1900-10hp-revision-b1-support-openwrt-firmware/131841/32
for putting me on the right track for this problem

Error that is being fixed - running fw_printenv results in:
"Warning: Bad CRC, using default environment"
and not showing boardmodel

Workaround, manually changing /etc/fw_env.config to
"/dev/mtd1 0x0 0x400 0x10000"

Signed-off-by: Klaas Demter <psychic-stool-cozy@duck.com>
Link: https://github.com/openwrt/openwrt/pull/17920
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>