board: fix SIGTERM handling in wan-monitor at shutdown

When the shell is blocked on sleep, signal handlers don't execute until
the command completes.  This caused the script to ignore SIGTERM at
shutdown, forcing Finit to wait and eventually SIGKILL it instead.

Fix by running sleep in the background and using wait, which is
interruptible by signals.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

board/aarch64/rootfs/usr/sbin/wan-monitor.sh

@@ -20,6 +20,7 @@ cleanup()
 {
     rm -f "$LED_FILE"
     rm -f "$PID_FILE"
+    kill %% 2>/dev/null
     exit 0
 }
 
@@ -31,11 +32,13 @@ remaining_time=$((1800 - $(awk '{print int($1)}' /proc/uptime)))
 
 while [ "$remaining_time" -gt 0 ]; do
     check_wan
-    sleep 1
+    sleep 1 &
+    wait $!
     remaining_time=$((remaining_time - 1))
 done
 
 while :; do
     check_wan
-    sleep 5
+    sleep 5 &
+    wait $!
 done