ueventd: Add bootdevice symlink

Create the /dev/block/bootdevice symlink to point to the
boot device information based on kernel command line.
This decouples references to the boot device from the
actual device nodes (e.g. eMMC, UFS).

Change-Id: I5e5e3e07ff38c8002ac3f188df80284f1f47892b
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
Conflicts:
init/init.c