cros_generate_update_payload: Use up to 30 retries (30 minutes)

10 retries (10 minutes) isn't enough for some builders. Switch to 30.

BUG=chromium:353558
TEST=Local + trybot.

Change-Id: Id2ab5991c2da4033d8859712559932131f792f5a
Reviewed-on: https://chromium-review.googlesource.com/191914
Tested-by: Don Garrett <dgarrett@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
diff --git a/host/cros_generate_update_payload b/host/cros_generate_update_payload
index a57d967..1abc739 100755
--- a/host/cros_generate_update_payload
+++ b/host/cros_generate_update_payload
@@ -87,7 +87,7 @@
     return 0
   fi
 
-  for x in {1..10}; do
+  for x in {1..30}; do
     sudo umount -v "$mnt_point"
     ret=$?
     if [ ${ret} -ne 0 ]; then