Set a default for INTERVENE and update usage information

We need a default for INTERVENE so that the script
doesn't complain when the user doesn't specify one
on the command line.

Also, update the usage information to properly
reflect that the user can specify up to 3 for the
intervene setting.
Merge branch 'withUI' of https://github.com/technosopher/OpenWRT-fastbuild into withUI
Merge pull request #10 from hawkinswnaf/withUI

Set a default for INTERVENE and update usage information