updated: use rw_access for cmd and conf deletion

to avoid termination with exception on r/o RFS:

  File "/usr/lib/python2.7/dist-packages/elbepack/commands/updated.py", line 408
, in action_select
    rmtree (prefix + "conf")
  File "/usr/lib/python2.7/shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 247, in rmtree
    rmtree(fullname, ignore_errors, onerror)
  File "/usr/lib/python2.7/shutil.py", line 252, in rmtree
    onerror(os.remove, fullname, sys.exc_info())
  File "/usr/lib/python2.7/shutil.py", line 250, in rmtree
    os.remove(fullname)
OSError: [Errno 30] Read-only file system: '/var/cache/elbe/repos/elbe_updated_1
/conf/home/configs/my.cfg'

Signed-off-by: Manuel Traut <manut@linutronix.de>