hostapd: backport BSSID black/whitelists

hostapd: backport BSSID black/whitelists

This change adds the configuration options "bssid_whitelist" and
"bssid_blacklist" used to limit the AP selection of a network to a
specified (finite) set or discard certain APs.

This can be useful for environments where multiple networks operate
using the same SSID and roaming between those is not desired. It is also
useful to ignore a faulty or otherwise unwanted AP.

In many applications it is useful not just to enumerate a group of well
known access points, but to use a address/mask notation to match an
entire set of addresses (ca:ff:ee:00:00:00/ff:ff:ff:00:00:00).

This is especially useful if an OpenWrt device with two radios is used to
retransmit the same network (one in AP mode for other clients, one as STA for
the uplink); the following configuration prevents the device from associating
with itself, given that the own AP to be avoided is using the bssid
'C0:FF:EE:D0:0D:42':

config wifi-iface
	option device 'radio2'
	option network 'uplink'
	option mode 'sta'
	option ssid 'MyNetwork'
	option encryption 'none'
	list bssid_blacklist 'C0:FF:EE:D0:0D:42/00:FF:FF:FF:FF:FF'

This change consists of the following cherry-picked upstream commits:

b3d6a0a8259002448a29f14855d58fe0a624ab76
b83e455451a875ba233b3b8ac29aff8b62f064f2
79cd993a623e101952b81fa6a29c674cd858504f
(squashed to implement bssid_{white,black}lists)

0047306bc9ab7d46e8cc22ff9a3e876c47626473
(Add os_snprintf_error() helper)

Signed-off-by: Stefan Tomanek <stefan.tomanek+openwrt@wertarbyte.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44438 3c298f89-4303-0410-b956-a3cf2f4a3e73