packaged-staging.bbclass: use 'os.unlink()' instead of spawning 'rm'

packaged-staging.bbclass: use 'os.unlink()' instead of spawning 'rm'

Doing a '-c clean' operation on a staged package with very much files
(e.g. glibc) took several minutes because

* every removed file was reported
* an 'rm' instance was spawned for every file

This patch uses the native 'os.unlink()' method for removing files and
reports only the removed root directory instead of the single files.

Based upon maillist discussion, reporting happens with 'debug' level
instead of 'note' one, and only error conditions due to non-existing
files will be ignored.  Other (e.g. permission denied) errors will now
abort the build while they were silently ignored previously.

Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Signed-off-by: Enrico Scholz <enrico.scholz@sigma-chemnitz.de>
Acked-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Acked-by: Tom Rini <tom_rini@mentor.com>
Signed-off-by: Tom Rini <tom_rini@mentor.com>