This commit:
1) seed /dev/urandom with the saved seeds as early as possible
   (see /lib/preinit/81_urandom_seed)
2) save a seed at /etc/urandom.seed if it doesn't exists
3) save a new seed each boot at "system.@system[0].urandom_seed"
   (see /etc/init.d/urandom_seed)

We use getrandom() so we are sure /dev/urandom pool is initialized

Seed size is 512 bytes (ie /proc/sys/kernel/random/poolsize / 8)
it's the same size as in ubuntu 14.04 and all systemd systems

Seeding /dev/urandom doesn't change entropy estimation, so we still have
"random: ubus urandom read with 4 bits of entropy available"
messages in the logs, but we can now ignore them if
after "urandom-seed: Seeding with ..." message

Saving a new seed on each boot is disabled by default to avoid too much
writes without user consent

v2: log preinit messages to /dev/kmsg
v3: use non generic function name for logging, as /lib/preinit/ files
    are all sourced together in /etc/preinit
v4: after a lot of discussion on the ML, use a uci config param
v5: config param is now the path of the seed

Signed-off-by: Etienne CHAMPETIER <champetier.etienne@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>