Increase the timeout for test payload generation

BUG=chromium:352707
TEST=Check if the stumpy_moblab buildbot pass the test payload
generation phase

Change-Id: I722297bae2124cbc90dc41cb33bb6869f20f6b4f
Reviewed-on: https://chromium-review.googlesource.com/190455
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Tested-by: Jay Kim <yongjaek@chromium.org>
Commit-Queue: Jay Kim <yongjaek@chromium.org>
diff --git a/lib/constants.py b/lib/constants.py
index f684452..b977d94 100644
--- a/lib/constants.py
+++ b/lib/constants.py
@@ -17,4 +17,4 @@
 
 CROSUTILS_LIB_DIR = os.path.join(CROSUTILS_DIR, 'lib')
 
-MAX_TIMEOUT_SECONDS = 2400
+MAX_TIMEOUT_SECONDS = 4800