ueventd: deprecate paths without /etc/

ueventd.rc scripts belong in the /etc/ directory of their given
partition, not the root of the partition.  This can cause problems,
especially since Android.bp cannot write to the root directly, forcing
vendors to use Android.mk for these files.  Note that
/system/etc/ueventd.rc moved long ago.

Test: Tree-hugger
Change-Id: I2dcaafc3c3f687f76ab6bc38af979c8b43346db0