hostapd: correctly handle macfile uci option

Make hostapd.sh correctly handle the macfile uci option.

Such option specifies the macfile name to pass into the
hostapd configuration file. Moreover, if a maclist option
has been specified, copy the macfile before appending new
entries.

Signed-off-by: Antonio Quartulli <antonio@open-mesh.com>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36944 3c298f89-4303-0410-b956-a3cf2f4a3e73