image.bbclass: avoid boot error on read-only systemd image

image.bbclass: avoid boot error on read-only systemd image

New version of systemd implements a new feature of updating /etc
or /var when needed at boot. For details, please see link below.

Opointer.de/blog/projects/stateless.html

For now, at boot time, the systemd-sysusers.service would update user
database files (/etc/passwd, /etc/group, etc.) according to the configuration
files under /usr/lib/sysusers.d. This step is necessary for other systemd
services to work correctly. Examples of such services are systemd-resolved
and systemd-tmpfiles-setup.

The problem is that on a read-only file system, that is, if /etc is read-only,
the user database files could not be updated, causing failures of services.

This patch fixes this problem by adding users/groups at rootfs time.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>