I haven't used Radxa SBCs, but I did some development for RK3188 GNU/Linux support half an year ago. At the time the hardware video encoders and decoders were not supported on GNU/Linux, but worked ok on Android (with libstagefright support I think).

I've added XV support[0] for video resizing and colorspace conversion, after which mplayer compiled with NEON support and with multithreading could decode in software pretty much any video I've thrown at it.

Things could have significantly improved since then, but I've been out of the loop.

However, I would strongly recommend just going for a Jetson-TK1 - I happen to be posting this from one. You get a fairly comprehensive reference manual and NVIDIA fully supports GNU/Linux (albeit with proprietary Xorg drivers at the time), the ARM cores (Cortex-A15 @ 2.3GHz) are much faster, the GPU supports full OpenGL (as opposed to OpenGL ES) and NVIDIA and the community are working on a fully open source driver stack (with nouveau).

[0] https://github.com/lgeek/xf86-video-fbturbo - very much abandoned alpha-quality software