Makefile: Add a target no-obs and its clean_no-obs counterpart.

build-scripts/*:
- Simplify the scripts and add a variable for the path of the current tarball.
- Add new variables and improve the genericization.
build-scripts/obs.sh: Delete .install.cfg in the directory where builtout is run.

templates/*: Improve the templates system: it is no longer needed to modify the templates to build another SR.
templates/Makefile.in: Improve the target management.