package_manager.py: fix bitbake package-index failed

package_manager.py: fix bitbake package-index failed

Previously the following commit in oe-core move RPM metadata
from DEPLOY_DIR to WORKDIR.
-----------
commit a92c196449c516fe51786d429078bbb1213bb029
Author: Stephano Cetola <stephano.cetola@linux.intel.com>
Date:   Wed Aug 10 13:03:16 2016 -0700

    Allow for simultaneous do_rootfs tasks with rpm

    Give each rootfs its own RPM channel to use.  This puts the RPM metadata
    in a private subdirectory of $WORKDIR, rather than living in DEPLOY_DIR
    where other tasks may race with it.
-----------

In the modification of 'class RpmIndexer, it should not
directly set arch_dir with WORKDIR. It caused 'bitbake
package-index' could not work correctly.

Assign WORKDIR as input parameter at RpmIndexer initial time
could fix the issue.

(From OE-Core rev: 3c8c8501d0a19b566a94a9e06afe40642b444958)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>