firmware: extract prism54-firmware into own package

firmware: extract prism54-firmware into own package

Instead of downloading the firmware for prism54 devices in the
wireless.mk do it in an extra package Makefile. To ship the complete
source code Intel ships our modified OpenWrt/LEDE + the content of the
dl directory. We do not want to have any files in the dl/ directory
which are not needed to build our images. The prism54 gets downloaded
every time independently of building kmod-net-prism54 or not. When it
is in a own package it only gets downloaded when the firmware package
is selected.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>