fwget: simplify adding firmware images to pkg to install

Rather than using echo to return the firmware package name, call a
new function (addpkg) which will also deal with (i) no leading space
and (ii) remove duplicates (as some devices have dual-wifi-cards).
In addition we won't have a line break when having multiple packages.

While here also do not call pkg(8) anymore if there is no package to
install and use the correct variable to install all and not just the
last found package.

Reviewed by: manu, bapt
Differential Revision: https://reviews.freebsd.org/D40071

fwget: Use BSD-2-Clause instead of BSD-2-Clause-FreeBSD

Reported by: bz
Sponsored by: Beckhoff Automation GmbH & Co. KG

fwget: Fix a few kmod name

The -kmod part was forgot in a few names for Intel firmware.

Fixes: d198b8774d2c ("fwget: Introduce new utility")
Reported by: bz
Sponsored by: Beckhoff Automation GmbH & Co. KG

fwget: Introduce new utility

This script's goal is to check the system for peripherals that needs
firmware and install the needed packages for them.
For now it only support pci subsystem and only video classes for AMD
and Intel GPUs.

Reviewed by: bapt
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D39825