CPack: re-use paths of tools discovered by CMakeFindBinUtils

CMakeFindBinUtils.cmake sets CMAKE_<TOOL> variables that in cross-compilation environments point to the toolchain specific executables: e.g. CMAKE_OBJCOPY might point to <path_to_toolchain>/arm-poky-linux-gnueabi-objcopy.

However, such variables are not exposed to CPack. This merge request is to let CPack internal modules re-use (a subset) of the already defined CMAKE_ variables.

I am not sure whether this is best method to do so, I guess that another method might be to let the internal CPack module use the HINTS ${_CMAKE_TOOLCHAIN_LOCATION} find option as the CMakeFindBindUtils.cmake already does.

Also, please consider that I am new to cmake development and I might not have considered all the implications. Hope this helps anyway.