Mark targets using kernel 3.18 as source-only

Mark targets using kernel 3.18 as source-only

We're past v17.01.0-rc2 phase and these few targets are stuck at 3.18
kernel. We obviously don't want to have targets like this built for the
release, especially with 3.18 being EOL.

It may be not too late for bringing these targets back for the next
minor release, so just mark them as source-only instead of dropping
completely.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>