Move init and ueventd scripts from / to /system/etc

There is no reason for these scripts to continue to exist in /, when
they are better suited for /system/etc.  There are problems keeping
them at / as well, particularly that they cannot be updated with
overlayfs.

Bug: 131087886
Bug: 140313207
Test: build/boot
Merged-In: I043d9a02ba588ca37ceba2c4e28ed631792b2586
Change-Id: I043d9a02ba588ca37ceba2c4e28ed631792b2586