hostapd: add support for static airtime policy configuration

hostapd: add support for static airtime policy configuration

* Add support for passing airtime_sta_weight into hostapd configuration.
* Since that commit it is possible to configure station weights. Set higher
  value for larger airtime share, lower for smaller share.

I have tested this functionality by modyfing /etc/config/wireless to:

config wifi-device 'radio0'
	...
        option airtime_mode '1'

config wifi-iface 'default_radio0'
	...
        list airtime_sta_weight '01:02:03:04:05:06 1024'

Now, when the station associates with the access point it has been assigned
a higher weight value.
root@OpenWrt:~# cat /sys/kernel/debug/ieee80211/phy0/netdev\:wlan0/stations/01\:02\:03\:04\:05\:06/airtime
RX: 12656 us
TX: 10617 us
Weight: 1024
Deficit: VO: -2075 us VI: 256 us BE: -206 us BK: 256 us

[MAC address has been changed into a dummy one.]

Signed-off-by: Dobroslaw Kijowski <dobo90@gmail.com>