interface: add interface dependency option

[legale]

Some interfaces run on top of the network provided by another interface. A typical case is a VXLAN or IPIP tunnel over a WireGuard tunnel. Without an explicit dependency, netifd can start configuring the upper interface before the lower one is up, which makes startup order fragile and pushes retry logic into protocol scripts.

Add an optional depends string for interfaces. When it is not set, netifd keeps the current behavior. When it is set, netifd delays setup until all listed interfaces are up. The dependency graph is validated to reject self-references and cycles, and autostart interfaces blocked by dependencies are retried when a dependency comes up later.

Keep the implementation narrow: only gate interface setup order. Do not add a separate readiness state, protocol-side ready/not-ready notifications, extra ubus events, or periodic reconcile retries.

Example:

config interface 'lan'
option proto 'dhcp'
option device 'br-lan'
option force_link '1'

config interface 'wg1'
option proto 'wireguard'
option force_link '1'
option depends 'lan'

config interface 'wan_vx3'
option proto 'vxlan'
option peeraddr '100.100.2.1'
option vid '3'
option depends 'wg1'

With this configuration, netifd will not start wg1 until lan is up, and will not start wan_vx3 until wg1 is up. If wan_vx3 is reached first in the normal startup walk, setup is deferred and retried automatically after the lower interface becomes available.

Observed netifd log:

Interface 'lan' is now up
Interface 'wg1' retrying setup after dependencies are up
Interface 'wg1' is setting up now
Interface 'wg1' is now up
Interface 'wan_vx3' retrying setup after dependencies are up
Interface 'wan_vx3' is setting up now
Interface 'wan_vx3' is now up

This shows the intended ordering: lan comes up first, then wg1, then the VXLAN interface that depends on wg1.