Add functions to generate an EUI based on the netdev ID.
This will always return the same EUI for the same netdev, so it is
stable across netdev resets.
This does two things:
The documentation of `luid_get()` is wrong, or at least confusing.
It talks about
> an 8-bit incrementing counter value into the most significant byte
while the implementation does
((uint8_t *)buf)[0] ^= lastused++; // 0 is LSB!
Now it could be argued that the intention was that the ID is supposed
to be used in Big Endian contexts and that was an omission, however
to keep everyone's sanity, let's keep it simple and just state that this
actually changes the LSB.
Also add a `luid_get_lb()` function that does the same, but modifies the
most significant byte - or the last byte if looking at the index.
This can then be used directly by e.g. #13743
Stated the obvious in the doc: Altering the LUID returned by luid_get() breaks
the guarantee that it is locally unique, as it than could collide with other
LUIDs returned by luid_get().
