net/wifi-firmware-kmod: allow ONLY_FOR_ARCHS to be set by child ports

[bz]

Summary

mtw(4) is built along USB for all architectures so we need firmware
on all architectures -- unlike the LinuxKPI based wireless drivers
which are restricted by default.

Allow ONLY_FOR_ARCHS to be set in a child port; maintaing a full list
of supported ARCHes seems unreasonable though so allow NOT_FOR_ARCHS
to disable the ONLY_FOR_ARCHS logic in Makefile.inc. That way
wifi-firmware-mt7601u-kmod can define an empty NOT_FOR_ARCHS and
it should build on all ARCHes.

Sponsored by: The FreeBSD Foundation

[bz]

rebase