OPENCV_CONFIG_FILE_INCLUDE_DIR

It is used for cvconfig.h only:

ocv_update(OPENCV_CONFIG_FILE_INCLUDE_DIR "${CMAKE_BINARY_DIR}/" CACHE PATH "Where to create the platform-dependant cvconfig.h")

Going in this way would increase mess of misused variables.

Purpose of these changes are unclear and it is not stated in PR's description.

It is used for cvconfig.h only

I don't think this is for cvconfig.h only. Actually, in cmake/OpenCVGenHeaders.cmake, there is a line for generating the OpenCV modules which uses the value:

configure_file("${OpenCV_SOURCE_DIR}/cmake/templates/opencv_modules.hpp.in" "${OPENCV_CONFIG_FILE_INCLUDE_DIR}/opencv2/opencv_modules.hpp")

As the variable name is indicating this is the directory for config file(s), I think custom_hal.hpp and opencv_data_config.hpp can both be considered as config files as well, just like the opencv_modules.hpp.

I don't think this will increase any mess. It's more like making the config files generation more consistent and more flexible instead of having two potentially different locations.

If this is not intending to be a configurable option, my suggestion would be changing it to a normal variable. The cache value will be shadowed by the normal variable with the same name and will probably result in CMake generation failure.

Referencing #6098 as this is the proposed solution.