init: Add vendor-specific initialization hooks

Allow optional vendor-specific initializations
within init.  This can be used for runtime
initialization setup that init rc scripts do
not support.

Change-Id: I7623a0d59b18f9ec8e3623958e2f7ccd72b877bf
(cherry picked from commit a36f7998a135a63934465889f089f96deefe7c42)
(cherry picked from commit 2ce27688b13bbc9a2237442ec2d6a554325ef658)
(cherry picked from commit 8e4dc4f79011bf0a8832e8d1251fc17b3e542057)