meta/lib/oe/rootfs.py: clean up dnf cache after creating an image

It contains cached metadata for a transient repository that is used
only when creating images on the host, and so is of no use on target
images. Dnf will recreate the cache on target when needed.

(From OE-Core rev: 89ddf2516b713e6622df95ab391423b033d4271a)

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/rootfs.py

@@ -525,7 +525,8 @@ class RpmRootfs(Rootfs):
             self.pm.save_rpmpostinst(pkg)
 
     def _cleanup(self):
-        pass
+        self.pm._invoke_dnf(["clean", "all"])
+
 
 class DpkgOpkgRootfs(Rootfs):
     def __init__(self, d, progress_reporter=None, logcatcher=None):