Use KF5_HOST_TOOLING to avoid patching KConfig

* Makefile.am (EXTRA_DIST): Remove patch.
* README: Document new host dependencies
* configure.ac: Check for kf5_host_tooling.
* src/toolchain.cmake.in: Set KF5_HOST_TOOLING.

Thanks to the android port there is no need to patch
KConfig to properly find the kconfig compiler of the
host system. Only KF5_HOST_TOOLING needs to be set.
While this creates additional build depdendcies its
better then to patch in a different way.