image.bbclass: search both rootfs and native staging for passwd files
When pseudo is configured to disallow fallback to the build host
/etc/hosts and /etc/group, the selection of ${IMAGE_ROOT} for
PSEUDO_PASSWD is insufficient as the necessary files will not be
available until base-passwd has been installed and its pkg_postinst
script run. Fall back to the ${STAGING_DIR_NATIVE} version of those
files until the rootfs versions are available. (The native copies are
never modified by the build; the ones in ${STAGING_DIR_TARGET} are
updated and may contain settings not consistent with what would be
created by post-install useradd/groupadd commands invoked in the image
rootfs.
