User overlays are working fine on this boards (I have already tested on rock64).
Added the possibility to use on rockchip64 based boards.
Changed "if" to "case" to facilitate adding more supported boards.
Current version only allows user-added overlays for 'dev' kernels. Excluding 'legacy' kernels should not be necessary anymore since <4 kernels are not supported anymore.
Ubuntu Bionic and newer Debian distributions have new enough dtc for
compiling overlays, so try to use distribution provided compiler if
kernel headers are not installed