Fix /init -> /system/bin/init symlink creation race

A symlink for TARGET_ROOT_OUT as a post install command of a package
that is also installed to TARGET_ROOT_OUT.  We hijack init.rc which
satisfies this requirement for this symlink.

Bug: 120402274
Bug: 120460755
Bug: 120509320
Bug: 120554662
Test: symlink is created
Test: make bootimage && ls -la $OUT/root/init
Change-Id: I6f1ac06ef152c36d7d7db4618d49a008338da39b