xserver-nodm-init: set USER and HOME

* when it's executed from init then HOME is set to '/'
* shadow/agetty set HOME fine and Xinit is doing the same, but for xinit Xsession it didn't work

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>