redbear-netctl: wait for interface to appear before waiting for DHCP address

wait_for_address assumed eth0 already existed and polled for an address with a
1s window -> on a fresh boot (smolnetd/driver-manager bring the interface up
async) it spun on a non-existent interface and logged "timed out waiting for
DHCP address on eth0". Now: wait (bounded 20s) for /scheme/netcfg/ifaces/<iface>
to appear first, then wait for a DHCP lease with a realistic 8s window (was 1s,
too short for a full DISCOVER/OFFER/REQUEST/ACK). Both bounded + oneshot_async so
boot never blocks. Pairs with the smolnetd/dhcpd async-NIC-attach fix.

local/recipes/system/redbear-netctl/source/src/main.rs

@@ -542,7 +542,10 @@ fn dhcp_wait_timeout() -> Duration {
         .ok()
         .and_then(|value| value.parse::<u64>().ok())
         .map(Duration::from_millis)
-        .unwrap_or_else(|| Duration::from_millis(1000))
+        // 8s: a real DHCP DISCOVER/OFFER/REQUEST/ACK handshake over a freshly
+        // brought-up link needs more than the old 1s (which expired before the
+        // lease ever arrived).
+        .unwrap_or_else(|| Duration::from_millis(8000))
 }
 
 fn dhcp_poll_interval() -> Duration {
@@ -553,10 +556,42 @@ fn dhcp_poll_interval() -> Duration {
         .unwrap_or_else(|| Duration::from_millis(50))
 }
 
+fn iface_appear_timeout() -> Duration {
+    env::var("REDBEAR_NETCTL_IFACE_WAIT_MS")
+        .ok()
+        .and_then(|value| value.parse::<u64>().ok())
+        .map(Duration::from_millis)
+        .unwrap_or_else(|| Duration::from_millis(20_000))
+}
+
+fn interface_exists(interface: &str) -> bool {
+    netcfg_root()
+        .join("ifaces")
+        .join(interface)
+        .join("mac")
+        .exists()
+}
+
 fn wait_for_address(interface: &str) -> Result<(), String> {
-    let deadline = Instant::now() + dhcp_wait_timeout();
     let poll = dhcp_poll_interval();
 
+    // The NIC driver + smolnetd bring /scheme/netcfg/ifaces/<iface> up
+    // ASYNCHRONOUSLY (driver-manager attaches drivers after this service
+    // starts), so on a fresh boot the interface may not exist yet. Wait for it
+    // to appear FIRST, then for a DHCP-assigned address. Both bounded so a
+    // NIC-less / no-DHCP machine still returns (this runs oneshot_async, so the
+    // wait never blocks boot/login).
+    let iface_deadline = Instant::now() + iface_appear_timeout();
+    while !interface_exists(interface) {
+        if Instant::now() >= iface_deadline {
+            return Err(format!(
+                "interface {interface} never appeared in /scheme/netcfg (no NIC, or its driver failed to attach)"
+            ));
+        }
+        thread::sleep(poll);
+    }
+
+    let deadline = Instant::now() + dhcp_wait_timeout();
     loop {
         match current_addr(interface).as_deref() {
             Some(addr) if addr != "Not configured" && !addr.is_empty() => return Ok(()),