net/wifi-firmware-kmod: add a release flavor

Add a flavor for a selection of wireless firmware packages to put
onto a relase media. There is no point shipping firmware for the
installer if there is no active driver for them yet.

Sponsored by: The FreeBSD Foundation
Reviewed by: jrm
Differential Revision: https://reviews.freebsd.org/D47141

(cherry picked from commit a4a64114c919e53333f51960c3fdac0c9c653057)