init.rc: mix device-specific data into the Linux RNG

Mix the contents of /proc/cmdline and /default.prop
into /dev/urandom. /proc/cmdline often contains
androidboot.serialno, a device-specific unique
identifier. Similarly, /default.prop contains the
build fingerprint and timestamp, which vary between
device families.

Change-Id: I8803b38c7089b2a1217b99a7c1808b29a3b138cf