During development, often a blank root password is used for the embedded Linux target system. However, when deploying an embedded Linux system, often there is a requirement to set the root password to something non-obvious. One way to do this is boot the system, and change the password using the passwd command. Then copy the password hash from the /etc/shadow file into the below line in your image recipe.
ROOTFS_POSTPROCESS_COMMAND += "\
sed 's%^root:[^:]*:%root:password_hash_from_etc_shadow:%' \
< ${IMAGE_ROOTFS}/etc/shadow \
> ${IMAGE_ROOTFS}/etc/shadow.new;\
mv ${IMAGE_ROOTFS}/etc/shadow.new ${IMAGE_ROOTFS}/etc/shadow ;"
The ROOTFS_POSTPROCESS_COMMAND is useful for simple modifications like this to the rootfs image.