net/wifi-firmware-*: use metaport for "base" and switch to Makefile.inc

Rename the wifi-firmware-base to wifi-firmware-kmod (similar to
gpu-firmware-mod). The former was never build so technically it
is no move. Make the new subdir a metaport including all
wireless firmware.
Move the old Makefile to Makefile.inc and adjust the child ports
accordingly.

This solves two problems: (1) the base port is now buildable and (2)
there is a single port to depend on to add all the needed firmware
files to, e.g., put on a release media.

Suggested by: amdmi3
PR: 270742
Reviewed by: ehaupt, jrm
Approved by: ehaupt, jrm
Differential Revision: https://reviews.freebsd.org/D40014