Add RNG implementation

[sirkrypt0]

Currently, we do not provide any RNG implementation in software.

It may be useful to add one, which, however, would require some sort of entropy function. This must be implemented by the user of the library for the respective board.

See the following section in the code:

libmicrofido2/src/random.c

Lines 14 to 17 in 1916feb

	int fido_get_random(void *buf, size_t len) {
	// TODO: Implement randomness here according to the standard.
	return 0;
	}

[felix-gohla]

Also add hardware RNG where supported. 😊

[felix-gohla]

Also think of disabling USE_SOFTWARE_RNG in the cmake configuration, as noted in #34.