chrome_cr50: match 26 characters in the board id

There are exactly 26 characters in the board id string. This changes the
regex to ensure the test matches all of them.

BUG=none
TEST=none

Change-Id: Ib2b8bfcb0bcec9924f808ea10f076b0bda8bd81e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2115192
Tested-by: Mary Ruthven <mruthven@chromium.org>
Reviewed-by: Aadit Maheshkumar Modi <modia@chromium.org>
Commit-Queue: Mary Ruthven <mruthven@chromium.org>
diff --git a/server/cros/servo/chrome_cr50.py b/server/cros/servo/chrome_cr50.py
index 9eec07c..3ae7402 100644
--- a/server/cros/servo/chrome_cr50.py
+++ b/server/cros/servo/chrome_cr50.py
@@ -66,7 +66,7 @@
     # Use the first group from ACTIVE_VERSION to match the active board id
     # partition.
     BID_ERROR = 'read_board_id: failed'
-    BID_FORMAT = ':\s+[a-f0-9:]+ '
+    BID_FORMAT = ':\s+[a-f0-9:]{26} '
     ACTIVE_BID = r'%s.*(\1%s|%s.*>)' % (ACTIVE_VERSION, BID_FORMAT,
             BID_ERROR)
     WAKE_CHAR = '\n\n'