Improve example benchmarking scripts

* Patching of OpenCV is removed from the build script, as the patch
  command does not always notice that a patch is already applied.
* Scripts can be run from any path.
* Description added for CUSTOM_CMAKE_OPTIONS in case of the build script.
* No need to specify KLEIDICV_PATH in case of the build script.
* Special env variables are removed from push.sh in favour of the
  official ADB ones.