android/x15-v2: fix the error on boot url specification

need to add '/' sign before boot_fit.img for the snapshot url
otherwise will get the error like here:
https://lkft.validation.linaro.org/scheduler/job/203283

Change-Id: If6a7ab7602850ccebbbc5820953bb00409a569fe
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/android/x15-v2/template-benchmarkpi.yaml b/android/x15-v2/template-benchmarkpi.yaml
index d099f34..7fa5b63 100644
--- a/android/x15-v2/template-benchmarkpi.yaml
+++ b/android/x15-v2/template-benchmarkpi.yaml
@@ -75,7 +75,7 @@
     namespace: droid
     images:
       boot:
-        url: "%%DOWNLOAD_URL%%boot_fit.img"
+        url: "%%DOWNLOAD_URL%%/boot_fit.img"
       cache:
         url: "%%ANDROID_CACHE%%"
       userdata: