Add KConfig recipe
Based on Johan's work branch. This also disables unit tests for all
frameworks, tests aren't executable for cross-builds anyway, and KConfig
tries to run the target kconfig_compiler there.
The native version can still further improved by allowing a QtCore-only
build of KConfig, as we are only interested in the kconfig_compiler.
Reviewed By: cordlandwehr
Differential Revision: https://phabricator.kde.org/D6987