MAR generation uses (mostly) hard-coded MAR update channel

Every MAR file has an associated MAR_CHANNEL_ID which consists of a triple in the form $BROWSER_NAME-$PUBLISHER-$CHANNEL. For tor-browser this is torbrowser-torproject-(alpha|release|nightly). At update-time the browser checks this string against its own stored in update-settings.ini

Adding base-browser to the mix causes a problem as the firefox build process will correctly update the update-settings.ini file to basebrowser-torproject-(alpha|release|nightly) but the MAR generation scripts use the hardcoded torbrowser-torproejct- prefix.