android x15-v2: update boot prompt to support AOSP P and master

use ":/" instead of "am57xevm:/"

Change-Id: I435966efa942d50b2d2b46ec8571b1e4a58508d4
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
diff --git a/android/x15-v2/template-javawhetstone.yaml b/android/x15-v2/template-javawhetstone.yaml
index 19116d4..ae51a4d 100644
--- a/android/x15-v2/template-javawhetstone.yaml
+++ b/android/x15-v2/template-javawhetstone.yaml
@@ -42,7 +42,7 @@
     namespace: tlxc
     prompts:
     - 'root@(.*):/#'
-    - 'am57xevm:/'
+    - ':/'
     timeout:
       minutes: 5
     method: lxc
@@ -89,7 +89,7 @@
     namespace: droid
     prompts:
     - 'root@(.*):/#'
-    - 'am57xevm:/'
+    - ':/'
     timeout:
       minutes: 15
     method: fastboot