scripts/dts: Remove unused variables and imports

Discovered with pylint3.

Use the placeholder name '_' for unproblematic unused variables. It's
what I'm used to, and pylint knows not to flag it.

Also improve the naming a bit in devicetree.py. If a key/value is known
to be a specific thing (like a node), then it's helpful to call it that
instead of something generic like "value".

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>