Adding OpenWrt support for MikroTik Chateau LTE12

Device: https://mikrotik.com/product/chateau_lte12

Current work can be found here: https://github.com/dchard/openwrt/commits/mikrotik_chateau_lte12

What works:

  • UART
  • Ethernet
  • LTE modem (Category 12.)

What does not work:

  • WIFI

For the WIFI part: MikroTik changed the BDF compression once again (to LZ77 this time). Hopefully @johnth can come up with a solution and update the platform driver soon.

As the Chateau LTE12 is a ROS7-only device (no simple way to downgrade to ROS6), the ability to build Routerboot7 compatible images is also needed. My branch already contains this part, based on the work of @johnth , tested all the way to ROS7.7 / RouterBOOT 7.7: both initramfs and sysupgrade images are working as expected, no need to downgrade or hack the device any other way.

Note: the modem is of course limited to Serial + MBIM by MikroTik, the AT command set is also somewhat limited. As the modem itself seems to be generic, hopefully it is possible to flash a generic firmware on it, which will provide QMI interface as well.

Once again, I want to thank @robimarko and @johnth for their hard work and help.