Add an option to corrupt all bytes in a firmware section

Current flashrom handler only provide an interface to corrupt a single
byte in a firmware section. Let's extend this to be able to corrupt all
bytes in order to improve test coverage.

BUG=chrome-os-partner:10265
TEST=Tested we can corrupt and restore firmware this way.

Change-Id: I20c56272440e393fa298561bd3c4c2f5ec9fe374
Reviewed-on: https://gerrit.chromium.org/gerrit/26396
Reviewed-by: Tom Wai-Hong Tam <waihong@chromium.org>
Commit-Ready: Vic Yang <victoryang@chromium.org>
Tested-by: Vic Yang <victoryang@chromium.org>