rtl8821au: new package

This patch adds the rtl8821au wifi driver. The driver is sourced from
ulli-kroll's work on GitHub, which includes various fixes and better LED
handling. Currently USB3 support is broken in this driver - it will
only connect to the EHCI controller in a USB3 port.

Signed-off-by: Christian Stewart <christian@paral.in>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>