As far as I know deimos is a set of official (?) bindings for common C libraries. I don't know dub's build process but I assume that if the build script would have worked for you it would have attempted to clone this repo https://github.com/D-Programming-Deimos/openssl (you can find more bindings if you go up one level). My gdc build hadn't finished yet when I had to leave for work this morning, but I'll try to build dub tonight, to see if I run into the same problems...
I've been using a Cubieboard A20, it's much better than a RPi as it has 1GB RAM, dual core armv7 and much better IO including on-board SATA for $50. The gcc /gdc build takes around 2 - 3 hours on that. You'll probably run into the same libphobos issues that I had, since there are many places where there are no ARM headers.
