Totaly stupid...
Can't login as root.
Have build an image with this:

require recipes-core/images/core-image-minimal.bb

# Use busybox as login manager
IMAGE_LOGIN_MANAGER = "busybox"

# Include minimum init and init scripts
IMAGE_DEV_MANAGER = "udev"
IMAGE_INIT_MANAGER = "sysvinit sysvinit-pidof"
IMAGE_INITSCRIPTS = ""

Any hint is appreciated...