hostapd: Add config_id parameter

hostapd: Add config_id parameter

Add a new configuration parameter: config_id.

If set, only do hostapd_clear_old() for the BSSes for which the
config_id changed.

This makes it possible to reconfigure specific BSSes on a radio,
without disconnecting clients connected to other, unchanged BSSes of
the same radio.

This patch adapted from a patch authored by John Crispin in the
OpenWrt repository:
https://git.openwrt.org/?p=openwrt/openwrt.git;a=blob;f=package/network/services/hostapd/patches/700-wifi-reload.patch;h=c5ba631a0fc02f70714cb081b42fcf6cb9694450;hb=60fb4c92b6b0d1582d31e02167b90b424185f3a2

Signed-off-by: Raphaël Mélotte <raphael.melotte@mind.be>