systemd: add hook to disable ldconfig.service

Since version 215, systemd installs a service to execute /sbin/ldconfig
when the sysinit target is reached.

Unfortunately, /sbin/ldconfig is not installed on a target built using a
glibc-based toolchain.

As this service is not mandatory, it is now disabled, thus avoiding a
warning in the journal.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>