Process rootfs one at a time

In the root_filesystems.py pipeline in the step:
```
@step
def match_not_analyzed_to_system_packages(self):
"""
Match not-yet-analyzed files to files already related to system packages.
"""
rootfs.match_not_analyzed(
self.project,
reference_status="system-package",
not_analyzed_status="",
)
self.next(self.match_not_analyzed_to_application_packages)
```

... we should consider doing it one rootfs at a time if there are several rootfs at once in the project e.g. `for rfs in self.root_filesystems:`...