ramips: invert wpad selection for mt7621

ramips: invert wpad selection for mt7621

For ramips/mt7621, the wpad-basic package is not selected by default,
but added for every device individually as needed.

While this might be technically correct if the SoC does not come with
a Wifi module, only 18 of 97 devices for that platform are set up
_without_ wpad-basic currently.

Therefore, it seems more convenient to add wpad-basic by default for
the subtarget and then just remove it for the 18 mentioned devices,
instead of having to add it for about 60 times instead.

This would also match the behavior of the 5 other subtargets, where
wpad-basic/wpad-mini is added by default as well, and thus be more
obvious to developers without detailed SoC knowledge.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>