init.rc: Fix entropy pool initialization

Init is responsible for initializing the /dev/[u]random entropy
pool, to ensure that high quality random numbers are available
to all Android processes. Previously Android mixed in data
from /system/etc/prop.default, which included properties which
differed on a device-by-device basis. That file no longer exists,
and init generates the following error at boot:

  init: Command 'copy /system/etc/prop.default /dev/urandom'
  action=init (/system/etc/init/hw/init.rc:106) took 0ms and
  failed: Could not read input file '/system/etc/prop.default':
  open() failed: No such file or directory

Instead of reading a property file, this change modifies init.rc
to read from /proc/bootconfig. The bootconfig file contains
per-device data which was previously on the command line, such
as androidboot.serialno. Additionally, it contains device-class
information such as androidboot.hardware,
androidboot.hardware.platform, and data which frequently churns
such as androidboot.vbmeta.digest. These help ensure that the
random number initialization is at least unique on a per
device-class basis.

Relying on /proc/bootconfig accomplishes the same goal that
relying on /system/etc/prop.default was intended to accomplish.
And /proc/bootconfig is in a more stable location than
/system/etc/prop.default, hopefully preventing future regressions.

Bug: 361624398
Test: Device boots and error above goes away
Change-Id: Id5c891e871217d62ff6517c5804b4acc2d723622