bitbake.conf: pseudo fall back to last-resort passwd files
Recipe packaging for the target requires permissions that are consistent
with meta/files/fs-perms.txt which specifies certain user and group
names. In the early parts of a target build base-passwd is not yet
available to provide the target /etc files used for user/group lookup.
Allow pseudo to fall-back to the last-resort files it installs if the
target ones aren't there yet.
