base-files, mac80211, broadcom-wl: use uci to populate wireless config

Previously, wifi detect simply dumped its generated wireless
configuration to STDOUT. A second step was needed to append
the configuration to /etc/config/wireless (or create it, if
it didn't exist).

With this patch, The wifi detection script will now use uci
to update the wireless configuration directly.

This patch also makes the initially created wifi-iface a
named section ('default_radio$X' for mac80211 and
'default_wl$X' for broadcom). With this change, uci will
not print the cfgHASH to STDOUT (which would now corrupt
the wireless configuration). It will also prevent adding
duplicated wifi interface configurations, if the wifi
configuration is run concurrently.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>