FLAVORS: Reorder so the default is first.

It is documented that the first FLAVOR is the default. Due to some use
of DEFAULT_VERSIONS and USES, the default FLAVOR may be determined
dynamically. Poudriere in particular expects the default to be first
and does the wrong thing for emacs ports currently [1]. This resolves
that and avoids other tools making the same mistake.

PR: 225659 comment #14
Approved by: portmgr (implicit)