networkd: Honor configured DHCP ClientIdentifier on link_update (#6622)

networkd: Honor configured DHCP ClientIdentifier on link_update (#6622)

We have an embedded board with a couple of ethernet ports. From the kernel
log, I can see that the ethernet drivers are obtaining their correct MAC
address, but for some reason, at first systemd-networkd doesn't see the
mac address for the ethernet port at the time that it looks at
dhcp_client_identifier configuration (it has 00:00:00:00:00:00 for mac).

Later on, systemd-networkd gets a link_update() call, and at this time, it
has the correct mac address for the ethernet port. However, in link_update()
the dhcp_client_identifier configuration is not being considered, and a call
to sd_dhcp_client_set_iaid_duid() is being done always