MFH: r368336

• Update to 3.0.18

Changes:

• Add a check for 3.1 repository and reject the build. 3.0 does not know how to handle 3.1's repository format. Downgrading is not supported at this point.
• Allow securelevel>=1 with USE_TMPFS=all
• Add a warning that DEVELOPER=yes is ignored in lieu of bulk -t/testport