e2build: introduce the build_process class

e2build: introduce the build_process class

Every result class has an associated build_process that knows how to
build results of this type.

If a new result class needs to extend the build process, it can inherhit
the class and overwrite methods or insert additional methods registering
them in the constructor.

Signed-off-by: Tobias Ulmer <tu@emlix.com>