CPU IDs may contain very little entropy. Often only a single byte
may change between two CPUs.
To mitigate this use a somewhat 'proper' hash algorithm (I chose DJB2
hash simply because it's small) and 'extend' it to the required output
size.
For 50 STM32 CPU IDs from the grenoble site, this produces a much more
random LUID output.
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
