gemini: add support for Teltonika RUT104

gemini: add support for Teltonika RUT104

Add support for Teltonika RUT104 3G HSUPA router.

This has been supported since about 20 years in the upstream Linux
kernel after initial contribution by Paulius Zaleckas from Teltonika.
It has some historical significance because I think it was one of the
first Teltonika Linux-based 3G routers.

Installation from scratch is done using the UART:

- UART soldering instructions with picture are available on the
  Link: (see bottom of committ message).
- First *diet down* your OpenWrt build as minimal as you can,
  I really mean this, delete everything you don't need. There
  is not much RAM to go around.
- Extract the "factory" firmare which is essentially just a tar.gz
  archive:
  tar xvfz openwrt-gemini-generic-teltonika_rut104-squashfs-factory.bin

From the RedBoot menu:

- Do NOT UNDER ANY CIRCUMSTANCE try to use the "upgrade firmare" (Z)
  alternative!
- Extract the three files zImage, rd.gz and hddapp.tgz from the archive.
- Put these three files in the root directory of your TFTP server
  (usually /var/lib/tftpboot)
- Hit 6 and set up the IP address for your device (e.g. 169.254.1.2 if
  you're using local link).
- Hit Y to "Upgrade Kernel", enter TFTP and your hosts IP number and
  type zImage. The kernel should upload and flash.
- Hit R to "Upgrade Ramdisk", enter TFTP and your hosts IP number and
  type rd.gz. The "ramdisk" (i.e. the second part of the kernel)
  should upload and flash.
- Hit A to "Upgrade Application", enter TFTP and your hosts IP number
  and type hddapp.tgz. The "application" (i.e. the root filesystem)
  should upload and flash.

This has a 1024KB Kernel partition, just extend the existing Make
functions to handle also this. The initramfs is 0x500000 instead
of 0x600000 for this one so add a parameter explicitly parameterizing
the initramfs size.

Mark non-default due to the small RAM and flash on this device.

I currently have no idea how to actually talk to the modem on this
thing but it is probably using the high-speed "modem UART" of the
Gemini. I'd be willing to help whoever wants to experiment with
it.

Link: https://dflund.se/~triad/krad/teltonika/
Link: https://github.com/openwrt/openwrt/pull/22045
Signed-off-by: Linus Walleij <linusw@kernel.org>