rpi-wifi-firmware: new package

Add firmware (NVRAM data) for the Raspberry Pi 3 and Zero W wifi module.

Notice that linux-firmware provides the main firmware
(brcmfmac43430-sdio.bin), but the module also needs board specific
configuration (NVRAM) data.

For the rpi, this data is available in the RPI firmware-nonfree git repo.
As this repo contains a lot of unrelated data (~70MB), simply download the
single NVRAM file instead of cloning the entire repo.

A similar approach is used by openwrt and opensuse:

https://dev.openwrt.org/browser/trunk/package/firmware/brcmfmac43430-firmware/Makefile?rev=49139
https://build.opensuse.org/package/view_file/hardware/bcm43xx-firmware/bcm43xx-firmware.spec?rev=b1628448b36c85abc9215eab4785ef29



[Peter: add _FIRMWARE_LICENSE as suggested by Yann]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>