py

Many error situations in U-Boot print the message:
    ### ERROR ### Please RESET the board ###

Add this to the list of bad patterns the test system detects. One
practical advantage of this change is to detect the case where sandbox
is told to use a particular DTB file, and the file cannot be opened.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>