Allow to override kconfig executable

This makes it possible to use kconfig_add_kcfg_files when cross compiling as it can be set in a cmake toolchain file to have it available in the build systems architecture.

The usecase for this is the Gpg4win build where we previously patched in compiled kcfg source files into Kleopatra. Ihis can instead be used to search for a HOST kconfig compiler in the configure stage and then set this as a variable in the cmake toolchain file explicitly. Otherwise finding this can be difficult because it is installed into libexec/kf5 prefix and usually not in the PATH.