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.

Bug: 26639863
Test: Tested on bullhead with androidboot.init_rc given a
non-existent .rc file which leads to expected boot failures.
Boot succeeds if androidboot.init_rc is not specified.

Change-Id: I2bca1cc3de6720feced041fe87266fb8afcce8b0