[yocto] core-image-minimal: no valid shadow password

[r10kindsofpeople]

I've found that if I take core-image-minimal and add a single package
(iproute2), I can no longer log in as root, getting the message
"login: no valid shadow password".  Adding the shadow packages didn't
seem to help.  Sounds like someone else encountered this back in May,
but there was no published resolution that I could find.

First tried under a Yocto Git master that is getting pretty old at
this point, then re-tried with a new download of 7.0.1 with the same
result.  Building for crownbay-noemgd.  Initially built
core-image-minimal using bitbake command line, and then added the
package(s) using hob, build package, build image.  Booting off an SDHC
card, if that matters.

Any suggestions?

John