hikey: Use ro.serialno to set usb_gadget string

Change-Id: I40bb46ef58599aec12694308cb8cbcc42dd28f5f
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
diff --git a/init.hikey.usb.rc b/init.hikey.usb.rc
index 9ab3de3..de54dbd 100644
--- a/init.hikey.usb.rc
+++ b/init.hikey.usb.rc
@@ -7,7 +7,7 @@
     write /config/usb_gadget/g1/bcdDevice 0x0223
     write /config/usb_gadget/g1/bcdUSB 0x0200
     mkdir /config/usb_gadget/g1/strings/0x409 0770
-    write /config/usb_gadget/g1/strings/0x409/serialnumber "0123456789"
+    write /config/usb_gadget/g1/strings/0x409/serialnumber ${ro.serialno}
     write /config/usb_gadget/g1/strings/0x409/manufacturer "HISILICON"
     write /config/usb_gadget/g1/strings/0x409/product "ADB Gadget"
     mkdir /config/usb_gadget/g1/functions/accessory.gs2