rng-tools: start rngd early enough to actually be useful

rng-tools: start rngd early enough to actually be useful

lighttpd starts at priority 50, but promptly calls getrandom() on
initialization (li_rand_reseed() and li_rand_device_bytes() from
server_init()). If /dev/urandom (which getrandom() uses by default)
doesn't have sufficient entropy, this will block.

Since Openwrt runs the startup scripts serially, this can block
initialization indefinitely.  I've seen 15-20 minutes typically.

Seeding the pool early on can quickly built sufficient entropy to
complete booting without blocking.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>