utils: sw_inventory: report unknown OS for software inventory

Users can create an image of a filesystem that contains no OS, therefore,
instead of rising an exception when no OS is detected, deliver a "unknown"
OS and an empty list of software.

src/utils/sw_inventory.py

@@ -126,6 +126,7 @@ def get_package_set(mountpoint):
         pkgset = _get_package_set_dpkg(dpkg_status_path)
         osname = getlinuxversion(osrelease)
     else:
-        raise ValueError(f'Cannot fetch software inventory at {mountpoint}')
+        pkgset = set()
+        osname = "unknown"
     # Legacy software inventory first element is the OS name
     return [osname] + list(pkgset)