Mk/bsd.port.mk: Add FLAVOR_SUB

If defined, for each flavors it will create for the non active one:
PLIST_SUB= FLAVORNAME="@comment " NO_FLAVOUNAME=""
and for the active one:
PLIST_SUB= FLAVORNAME="" NO_FLAVOUNAME="@comment "

Same for SUB_LIST

Reviewed by: manu, bdrewery, mat
Differential Revision: https://reviews.freebsd.org/D32495