kernel: Initialize RNG using CPU RNG and bootloader

kernel: Initialize RNG using CPU RNG and bootloader

This activates the following kernel options by default:
* CONFIG_RANDOM_TRUST_CPU
* CONFIG_RANDOM_TRUST_BOOTLOADER

With these option Linux will also use data from the CPU RNG e.g. RDRAND
and the bootloader to initialize the Linux RNG if such sources are
available.
These random bits are used in addition to the other sources, no other
sources are getting deactivated. I read that the Chacha mixer isn't
vulnerable to injected entropy, so this should not be a problem even if
these sources might inject bad random data.

The Linux kernel suggests to activate both options, Debian also
activates them. This does not increase kernel code size.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>