hostapd: Add optional support for hostapd own_ip_addr in wireless config

hostapd: Add optional support for hostapd own_ip_addr in wireless config

`own_ip_addr` is used by hostapd as NAS-IP-Address.
This is used to identify the AP that is requesting the authentication of the
user and could be used to define which AP's can authenticate users.
Some vendors implement only NAS-Identifier or NAS-IP-Address and not both.
This patch adds ownip as an optional parameter in /etc/config/wireless.

Signed-off-by: Thomas Wouters <thomaswouters@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40934 3c298f89-4303-0410-b956-a3cf2f4a3e73