Enable MIPS and PowerPC booting

[nathanchance]

Split off from #2.

Blocked on tuxmake: https://gitlab.com/Linaro/tuxmake/-/issues/109

[nathanchance]

tuxmake has support now, which I have verified locally: https://gitlab.com/Linaro/tuxmake/-/merge_requests/100

We just need tuxsuite support now: https://gitlab.com/Linaro/tuxsuite/-/issues/95