package_manager.py: fix rootfs failure with multilib enabled

With the current code, if we use debian package backend and enable
multilib support, the do_rootfs process would always fail with error
messages like below.

    E: Unable to locate package packagegroup-core-boot

This patch fixes the above problem.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>