The "user=" boot option does not work (tinycore 1.0).
In /etc/init.d/rcS, line 64 reads:
/bin/adduser -s /bin/sh -g staff -d "/home/$USER" "$USER"
It fails with the error message:
/bin/adduser: invalid option --d
I believe the option to create a user directory with busybox's adduser is "-h" instead of the familiar "-d" for useradd.
A snapshot is attached showing the result of booting with "tinycore user=foo pause"