asus-grouper: add wifi firmware to device build

This MR attempts to pick up where antonok's PR on Github left off, by sourcing the firmware from the AOSP firmware package on PMOS.

With this firmware I can connect to my AP using wpa_supplicant, ping servers and fetch files normally. However the driver also brings up the p2p0 interface on boot and it seems to conflict with wlan0. So to test the wireless we need to disable p2p0 and stop the wpa_supplicant service (because it attaches itself to p2p0 on boot):

$ sudo ip link set p2p0 down
$ sudo rc-service wpa_supplicant stop
$ wpa_passphrase <your ESSID> <your password> > ./wpa_config
$ sudo wpa_supplicant -B -Dnl80211 -iwlan0 -c./wpa_config
$ sudo udhcpc -i wlan0
$ ping postmarketos.org
PING postmarketos.org (188.166.143.8): 56 data bytes
64 bytes from 188.166.143.8: seq=0 ttl=42 time=41.604 ms
64 bytes from 188.166.143.8: seq=1 ttl=42 time=56.022 ms
64 bytes from 188.166.143.8: seq=2 ttl=42 time=54.911 ms
64 bytes from 188.166.143.8: seq=3 ttl=42 time=54.241 ms
...
$ sudo apk update
fetch http://postmarketos.brixit.nl/armhf/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/main/armhf/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/community/armhf/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/edge/testing/armhf/APKINDEX.tar.gz
v20190228-49-gbc0f9441b0 [http://dl-cdn.alpinelinux.org/alpine/edge/main]
v20190228-41-g6ee930cc6c [http://dl-cdn.alpinelinux.org/alpine/edge/community]
v20190228-48-g67283cdbf6 [http://dl-cdn.alpinelinux.org/alpine/edge/testing]
OK: 13694 distinct packages available

I'm still not sure about where the nvram settings file should go, though.