init: add ro.boot.init_rc

SoC vendors and ODMs need a way to run different init scripts under
different boot modes. This patch adds a new ro.boot.init_rc kernel
cmdline argument to support this.

This patch also changes late-init trigger. Now late-init is only
triggered in "normal" boot (where boot mode is not specified). This is
to make AOSP init.rc re-usable in other boot modes as the operations
in late-init are less common.

Bug: 26639863
Test: Tested on bullhead with
  androidboot.init_rc = {non-existent .rc file}
  and/or
  androidboot.init_rc = {none empty string}
  both of which lead to expected boot failures.
  Boot succeeds if androidboot.init_rc is not specified.
Change-Id: Ie5f08c3914f2f825ad3c401fde6741459668c523