MTD modification commands: add & prioritize

Additionally renames the new functions in add to be CamelCase style

BUG=chromium:221745
TEST=MTD version of run_cgpt_tests.sh passes
BRANCH=cros/embedded
CQ-DEPEND=CL:47172

Change-Id: Ic173f99d7ca4af025403ab11f37061c33c9d59ea
Reviewed-on: https://gerrit.chromium.org/gerrit/47173
Tested-by: Albert Chaulk <achaulk@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Queue: Albert Chaulk <achaulk@chromium.org>