device: fix potential truncation of DT-derived device names

While using the encoded path to a device tree node guarantees a unique
identifier for the corresponding device there is a limit on the number
of characters of that name that can be captured when looking up a
device by name from user mode, and the path can exceed that limit.

Synthesize a unique name from the node dependency ordinal instead, and
update the gen_defines script to record the name associated with the
full path in the extern declaration.

Add a build-time check that no device is created with a name that
violates the user mode requirement.

Also update the network device DTS helper functions to use the same
inference for dev_name and label that the real one does, since they
bypass the real one.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>