For my Raspberry Pi, I needed to support the rtl8821au driver for my wireless USB device. The following steps worked for me.
This is based on this article
pacman -S make dkms linux-raspberrypi-headers
git clone https://github.com/diederikdehaas/rtl8812AU.git
CONFIG_PLATFORM_I386_PC from y to n
CONFIG_PLATFORM_ARM_RPI from n to y
To use the driver and the device, several additional steps have to be followed, this is based on StackExchange.
pacman -S netctl dhclient
install -m640 examples/wireless-wpa wireless-home
Adopt above file to your needs
netctl start wireless-home
netctl enable wireless-home
After all those steps and another reboot, the wifi-device should work.