firmware: remove all firmware files available in kernel repo

[MilhouseVH]

This removes all firmware files that are available in the kernel repository (formerly x86-firmware) repo here.

The only files that need to be in this repo are those firmware files that don't exist anywhere else, or at least not in the kernel repo.

At this time, this is:

7ba1f3084a61a4bcf7b8c8396e8a4201  ./brcm/BCM20702A0-0a5c-21e8.hcd
7ba1f3084a61a4bcf7b8c8396e8a4201  ./brcm/fw-0a5c_21e8.hcd
7ba1f3084a61a4bcf7b8c8396e8a4201  ./fw-0a5c_21e8.hcd
99d4b41a23d6551090955cc42f92d3f4  ./brcm/BCM20702A0-0a5c-21ec.hcd
99d4b41a23d6551090955cc42f92d3f4  ./brcm/fw-0a5c_21ec.hcd
99d4b41a23d6551090955cc42f92d3f4  ./fw-0a5c_21ec.hcd
b95ee9032a723daa25ab9608a515e984  ./brcm/BCM43430A1.hcd

The first 6 files all appear to be variations of each other - I'd expect we can remove some of these (ie. 4 of them). Any suggestions?

The last file, brcm/BCM43430A1.hcd, is the RPi3 bluetooth firmware - I'm not sure if we need this here, as it's also installed by brcmfmac_sdio-firmware-rpi.

[chewitt]

firmware/intel has symlinks for files which have alternate names so you could do the same for the brcm folder. It won't save space in images (as squashfs de-dupes) but it's cleaner. I'd drop the Pi3 firmware if it's available from the other package.

[MilhouseVH]

Dropped the RPi0/RPi3 firmware, switched to sym links

[MilhouseVH]

Added 91606f0, see #2

[chewitt]

@MilhouseVH thanks for this.. let's see what breaks :)