systemd: add user for systemd-oomd if enabled

systemd-oomd.service has:

User=systemd-oom

So we need to create a static user for it, if its packageconfig
is enabled.

Signed-off-by: Luca Boccassi <luca.boccassi@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>