sanitycheck: Allow skipping the 'unrecognized section' test

Allow the user to disable the "unrecognized section" test. I can see
multiple use-cases for disabling the test.

If orphan sections exist and are dynamically or unpredictably named
the unrecognized section test will fail.

If out-of-tree sections exist, one might want to temporarily disable
the "unrecognized section" test until one has made it recognized.

The test is disabled through a CLI flag.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>