The 'user=' boot option is processed by tc-config which explicitly calls /bin/adduser to add the user record, but this symlink to busybox is missing in 9.0.3.gz and the user is not added.
The link /bin/adduser -> busybox exists in the x86 branch, and both branches have /usr/sbin/adduser -> busybox, but this isn't called by tc-config. /bin/addgroup is affected similarly.
Cheers,
Phil.