Keep entropy pool full

[CurlyMo]

For a proper discussion: https://github.com/xbianonpi/xbian/commit/cafa78a7a2097df03a57c866937d0596fc469eec

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And benchmarks on my XBian install:

Terminal

root@pi:~# time head -c 20m /dev/frandom > /dev/null

real 0m0.795s
user 0m0.020s
sys 0m0.770s
root@pi:~# time head -c 20m /dev/urandom > /dev/null

real 0m13.936s
user 0m0.010s
sys 0m13.790s
root@pi:~# time head -c 20m /dev/erandom > /dev/null

real 0m0.783s
user 0m0.020s
sys 0m0.760s
root@pi:~# time head -c 20m /dev/random > /dev/null

real 0m56.068s
user 0m0.400s
sys 0m50.690s