We have only been lucky so far that there hasn't been any compilation errors
caused by using the HOST platform config_*.h files also for the BUILD
platform. But as more and more information found out during configury is
stored in config_*.h files, sooner or later it will be a problem.

Move config/* to config_host/*. When not cross-compiling, the config_*.h files
in config_host expanded from config_*.h.in are all there is. When
cross-compiling, the generated config_*.h files for the BUILD platform are
put into a config_build folder.

Then use config_build when building the cross_toolset and config_host
otherwise.

Change-Id: I3c28cc83866bffd696b45d00dfb68e00bc2ae538