release: install wireless firmware onto disc1 and dvd

Wireless driver firmware is no longer added to the src tree.
In order to have wireless support in the installer for the new drivers
we install the firmware packages onto disc1 (and memstick) and dvd
if built on FreeBSD and NOPKG is not defined (to not break cross-builds
from Linux or OSX and to allow people to opt-out).

Sponsored by: The FreeBSD Foundation
Submitted by: cperciva (the orig. commands and where to place them)
Reviewed by: jrtc27
MFC after: 6 hours
Differential Revision: https://reviews.freebsd.org/D47407