wifi: Introduce 802.11ac support

This patch introduces 802.11ac support to base-files, mac80211 and hostapd. The
split of VHT160 in two 80 MHz bands is not yet supported, since it requires an
additional user supplied parameter for the channel of the second band.

Signed-off-by: Matti Laakso <malaakso@elisanet.fi>
Signed-off-by: Simon Wunderlich <simon@open-mesh.com>
[sven@open-mesh.com: Rebased patch, renamed hwmode to "ac",
fixed hostapd integration]
Signed-off-by: Sven Eckelmann <sven@open-mesh.com>