This is a little out of date. Device tree exists explicitly to avoid needing a machine file for your board. Bootloaders like uboot will load your kernel and dtb together, and the kernel uses the dtb to enumerate the hardware on the system.

Again, the problem isn't that the mainline kernel _couldnt_ support these boards, is that no one wants to put in the work to bring their implementations up to the standards of the mainline kernel.