This decision is probably out of your hands, but I'm happy to share a few words of caution. I've been the yocto distro maintainer at a large firm for about three years and I absolutely despise it. BMW released a slide deck about their experience with yocto, and it was mostly "it sucks." I read that recently and it matched my experience down to a t.
If you absolutely need third party commercial support, yocto is probably the only game in town, but if you don't, I would highly recommend looking at nix (which recently gained cross compile support) or buildroot.
I'm dead serious about avoiding yocto if at all possible, I've seen things I wish I could unsee. But that sounds about right for a military project.
EDIT: I put my email into my profile if you want to chat about yocto more in depth.
Right now, we are looking at moving away from vxWorks and onto a Yocto-built Linux with the RT patches to the scheduler. From a requirements standpoint, the RT patches meet our needs for "real time" since our timings require more of a soft real time than a hard one. There's also way more support out there for Linux, and I am also slightly biased in that I am more of a Linux guy than a vxWorks guy simply due to familiarity and ease of troubleshooting.
This latest news with vxWorks is great, however. I actually sent this to management because we are attempting to figure out what direction to take as our current version is reaching EOL.