Initialize the build-tools config as part of the master config
Right now the build tools config is only initialized when running when bootstrapping under the build tools bootstraper itself.
Given that the environment of a configuration requires to have knownledge about the build tools environment to correctly set search paths for binaries and shared library, the build-tools config should be initialized as part of the master config.
This allows in the config to correctly set LD_LIBRARY_PATH based on the build tools lib_suffix instead of hardcoded to 'lib' as it is right now.
This patch also allows to support other features such as allowing to run the build command for the build tools with a --build-tools switch, which is currently not possible.