lib/oe/package_manager: don't try to rm /var/lib/opkg

lib/oe/package_manager: don't try to rm /var/lib/opkg

As opkglibdir starts with a /, os.path.join will ignore
self.target_rootfs, leading to an attempt to remove /var/lib/opkg.

This only fails if it exists on the host, explaining why this remained
undiscovered for long.

Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit fc974977cea389f54e7fc7de7b1c8fd3d8bafe58)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>