Introduce the variable SOT_ROOT_DIR.

This indicates the root path to all sot repo:
src, install and stack.
The goal is to avoid wandering ~/devel/ROS_DEVEL_NAME paths.
Add a method to create a config file defining useful env. variables.
Small correction of the config.sh file generated.
Add the source repositories for the hrp4.
Add the possibility to remove the cmake cache.

It can be useful e.g. if the pkg-config files were changed.
Correct the path for hrp4 folders.
Add openhrp's pkg_config path.
Add the path to idh server.