Fix default python flavor wrt python version
There's a flaw in FLAVOR selection logic which makes python FLAVOR
default to py27 even with DEFAULT_VERSIONS=python=3.6. Fix this and
generate FLAVOR based on PYTHON_DEFAULT, PYTHON2_DEFAULT, PYTHON3_DEFAULT
(in that order) similar to how it's done in other parts of python.mk.
This does not affect FLAVOR in default setup (py27 remains), but
if python default version is modified, it will now properly affect FLAVOR.
Approved by: portmgr (antoine)
Differential Revision: D13326