brcm2708-gpu-fw: download zipped sources and switch to Hexxeh's git

brcm2708-gpu-fw: download zipped sources and switch to Hexxeh's git

* Use the gzipped download feature from github to get a tarball with a
  stable hash.
* Use Liam McLoughlin's git which has a subset of the original git's
  contents, dropping all the precompiled modules and kernels.

Reduces the download size from ~1.27 GiB to ~25 MiB.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36667 3c298f89-4303-0410-b956-a3cf2f4a3e73