treewide: gather DEVICE_VARS into one place

treewide: gather DEVICE_VARS into one place

Place DEVICE_VARS assignments at the top of the file or above Device/Default
to make them easier to find.

For ramips, remove redundant values already present in parent file.

Signed-off-by: Sungbo Eo <mans0n@gorani.run>
[do not touch ar71xx, extend commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>