hostapd: add support for Realtek driver

hostapd: add support for Realtek driver

Since kernel drivers for Realtek wireless chips use non-standard
interfaces, upstream hostapd does not support them. One have to apply
an external patch for hostapd to work with these chips. See:
https://github.com/pritambaral/hostapd-rtl871xdrv

A configuration option is added to enable support for Realtek chips,
and it's turned off by default.

Signed-off-by: Alexander Mukhin <alexander.i.mukhin@gmail.com>
Tested-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>