dhcp_get_filename.sh: Save boot server IP in a local file

The boot server IP is saved into /tmp/boot_server_ip. It can be useful
to get it to download files from this location in other scripts.

userspace/rootfs_override/wr/bin/dhcp_get_filename.sh

@@ -24,3 +24,8 @@ if [ -n "$boot_file" ]; then
     # received from the DHCP server.
     echo $boot_file > "$tmpdir"/dot-config_source_url
 fi
+
+# save IP@ of the boot server
+if [ -n "$siaddr" ]; then
+   echo $siaddr > "$tmpdir"/boot_server_ip
+fi