### What needs to get done Change the format to `$CRAFT_[<namespace>_]<partition>_PART_OVERLAY` to make it obvious these are part-specific env vars. ### Why it needs to get done Environment variables for partitions are available to overlay scripts, formatted as `$CRAFT_[<namespace>_]<partition>_OVERLAY`. However these variables points to a part-specific directory so their name is inconsistent with other part-specific env variables (ex. `$CRAFT_PART_INSTALL`, `$CRAFT_PART_SRC`), which may be misleading to users using these variables in different parts.