useradd.bbclass: set PSEUDO_PASSWD consistent with root directory
When installing into a sysroot this class examines $D/etc/passwd for
content, then invokes useradd to make changes. Under pseudo useradd
attempts to look up user information in directories specified by
$PSEUDO_PASSWD. For opkg multilib installs $D is not always the same as
$IMAGE_ROOT, and the user might already be in the IMAGE_ROOT files,
causing a failure during rootfs population.
Fix this by ensuring the files pseudo looks at when doing useradd stuff
are the same ones that useradd.bbclass will be manipulating.
