Add various FLAVOR related helpers.
To help with flavored ports, helpers are available. In these, replace <*> with the flavor name. *_PKGNAMEPREFIX *_PKGNAMESUFFIX *_PLIST *_DESCR will overwrite the variable. *_CONFLICTS *_CONFLICTS_BUILD *_CONFLICTS_INSTALL *_PKG_DEPENDS *_EXTRACT_DEPENDS *_PATCH_DEPENDS *_FETCH_DEPENDS *_BUILD_DEPENDS *_LIB_DEPENDS *_RUN_DEPENDS *_TEST_DEPENDS will append to the variable.
Reviewed by: portmgr
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D12483