treewide: use DEFAULT := n to disable non-broken devices

treewide: use DEFAULT := n to disable non-broken devices

This uses DEFAULT := n on a bunch of devices that previously were
"disabled" by commenting out TARGET_DEVICES. This will allow to
build them without having to modify the code, but they will not
be selected for default build or buildbots.

The change is applied to all devices that are not "broken", but suffer
from image site limitations or similar, or have been added in the past
but never confirmed to run on the device properly:

at91: at91-q5xr5:
  kernel image too big
  31aeae077482 ("at91: do not build image for at91-q5xr5")

bcm47xx: asus-rt-ac66u:
  disabled since it was added in 2015
  69aefc771fd8 ("brcm47xx: build images for Asus devices")

bcm47xx: netgear-wndr3400-vcna, netgear-wnr3500u, netgear-wnr3500-v2-vc
  added disabled in 2012, but never confirmed to work on devices
  5dec9dd3b200 ("brcm47xx: add code to generate images for some netgear
    devices")

bcm53xx: netgear-r8500
  added disabled: "start working on Netgear R8500"
  3b76c7cf0bdd ("bcm53xx: start working on Netgear R8500")

Signed-off-by: Petr Štetiar <ynezz@true.cz>
[limit to subset of devices, adjust commit message/title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>