See https://cmake.org/cmake/help/v3.0/variable/CMAKE_SYSROOT.html
See http://stackoverflow.com/questions/18900855/how-to-work-with-external-libraries-when-cross-compiling (about 1/2way down)

The workflow would then be this:

Provision ResinOS with all the OpenCV deps (incl python)
Copy root directory to computer, or mount the SD card
Point CMake to ResinOS root dir with -DCMAKE_SYSROOT
make install onto SD card