drtio: disable all destinations in gateware at startup

Otherwise, kernels fail to get a RTIODestinationUnreachable exception when attempting
to reach a DRTIO destination that has never been up.

artiq/firmware/libboard_artiq/drtio_routing.rs

@@ -97,3 +97,10 @@ pub fn interconnect_enable_all(routing_table: &RoutingTable, rank: u8) {
         interconnect_enable(routing_table, rank, i as u8);
     }
 }
+
+#[cfg(has_drtio_routing)]
+pub fn interconnect_disable_all() {
+    for i in 0..DEST_COUNT {
+        interconnect_disable(i as u8);
+    }
+}

artiq/firmware/runtime/main.rs

@@ -289,6 +289,8 @@ fn startup_ethernet() {
         drtio_routing::RoutingTable::default_empty()));
     let up_destinations = urc::Urc::new(RefCell::new(
         [false; drtio_routing::DEST_COUNT]));
+    #[cfg(has_drtio_routing)]
+    drtio_routing::interconnect_disable_all();
     let aux_mutex = sched::Mutex::new();
 
     let mut scheduler = sched::Scheduler::new();