cbfstool: test for duplicate files earlier

No need to read the file before bailing out.

Change-Id: Ida7226c6ec227e1105724cdb1e5a0927217a69c7
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/11212
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
1 file changed