"OpenDingux" and "RetroFW" are both linux-based MIPS distributions, but have evolved to be somewhat different. My understanding is RetroFW is a fork of an older Opendingux version. "Opendingux" proper only runs on three devices as of right now, the Dingoo A320, the GCW-Zero, and the RS-90.
http://www.treewalker.org/opendingux/ is usually the place to go for OpenDingux info, but seems to be down right now.
All the current JZ4760/B devices, such as the RG300 run on "RetroFW". You may have better luck searching for "RetroFW buildroot" and similar terms. If you're willing to use Discord, my understanding is the RetroFW devs have a big community there. I'm sure you can find a link to it somewhere around here, as I've seen it posted.
@gameblabla has ported a bunch of stuff to both Opendingux and RetroFW. I would recommend a quick browse through his post history, as I recall him giving advice on various toolchain things and such around the forums here.
