for details. Debootstrap is the tool that generates a "minimum bootable rootfs". You can use any existing debian install (even a non-mips architecture) to do the debootstrap.
You will need to build your own kernel. Check the OpenWRT project for patches, although only a very very few Ubiquiti devices (USG-3 for example) need kernel patches. For other devices (EdgeRouter-4) the OpenWRT packages make things nicer, like getting the network device names to match what's printed on the front of the case.
Put the kernel and rootfs on a USB stick, plug it into the router, attach the serial console (nice easy RJ45 jack on the front!) and boot. Once it's up you can migrate stuff to the internal soldered-down emmc.
You will need to build your own kernel. Check the OpenWRT project for patches, although only a very very few Ubiquiti devices (USG-3 for example) need kernel patches. For other devices (EdgeRouter-4) the OpenWRT packages make things nicer, like getting the network device names to match what's printed on the front of the case.
Put the kernel and rootfs on a USB stick, plug it into the router, attach the serial console (nice easy RJ45 jack on the front!) and boot. Once it's up you can migrate stuff to the internal soldered-down emmc.
Octeons are awesome.