package_rpm.bbclass: ensure base-passwd and shadow get installed first

package_rpm.bbclass: ensure base-passwd and shadow get installed first

When generating images, we need to make sure that base-passwd and
shadow get installed before other packages, which might need to create
custom user accounts.

Thanks to Richard Purdie for the initial version of this fix.

This fixes [YOCTO #2127]

Signed-off-by: Scott Garman <scott.a.garman@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>