add support for esp32c3 on-chip random generator to crypto/rand package.

[zdima]

The ESP32-C3 contains a true random number generator. This will allow utilize this for cryptographical operations, among other things.

[aykevl]

I think the current implementation is not safe against predictable numbers. Specifically:

When there is noise coming from the SAR ADC, the random number generator is fed with a 2-bit entropy in one clock cycle of RTC20M_CLK (20 MHz), which is generated from an internal RC oscillator (see Chapter 6 Reset and Clock for details). Thus, it is advisable to read the RNG_DATA_REG register at a maximum rate of 1 MHz to obtain the maximum entropy.

For details on how this can be implemented, see the esp_random function:

https://github.com/espressif/esp-idf/blob/v4.3/components/esp32c3/hw_random.c

That said, adding a RNG like this seems like a good idea (if implemented correctly).

[aykevl]

This duplicates the r variable, which is a bit confusing. Can you rename the r variable (and possibly other single letter variables) to something more readable?

[aykevl]

It appears that enabling random number generation is a bit more complicated than this:

https://github.com/espressif/esp-idf/blob/v4.3/components/bootloader_support/src/bootloader_random_esp32c3.c

I'm worried that it is not truly random without this complete initialization.

[zdima]

I will move it to draft until have move details on initialization and the #2325 with #2333 are move so I would have better access to register's fields.