bsd.port.mk: Do not pass FLAVOR in dependency make environment

[ross_ross-williams.net]

Summary

The change introduced in bug #256301 (review D30579), which prevents child make processes from receiving an empty FLAVOR variable when FLAVOR should be unset, has the side effect of allowing any FLAVOR already in the parent make process environment to propagate to the child.

This revision prevents the FLAVOR from the parent make from incorrectly propagating to the child during a recursive make.

[ross_ross-williams.net]

Wrong diff got uploaded.