netifd: use stable IAID for DHCPv4

netifd: use stable IAID for DHCPv4

Commit 9151c7015ed2 introduced support for the global DHCP DUID to
generate a RFC4361-style client identifier.
However, the IAID introduced in those changes is based on ifindex, which
is subject to changes and causes issues on environments requiring a stable
IAID.

This commit switches the IAID to a stable one based on MD5.

Fixes: 9151c7015ed2 ("netifd: use the global DHCP DUID for DHCPv4")
Link: https://github.com/openwrt/openwrt/pull/21489
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>