[Bug]: rockchip-rk3588.conf overrides OVERLAY_PREFIX

### What happened?

In file `config/sources/families/rockchip-rk3588.conf`

```
OVERLAY_PREFIX='rockchip-rk3588'
```

It must use default definition only, without overriding
```
OVERLAY_PREFIX="${OVERLAY_PREFIX:-"rockchip-rk3588"}" # default to 'rockchip-rk3588' if not set by board
```

### How to reproduce?

define custom `OVERLAY_PREFIX` in config/boards that depends on `rockchip-rk3588`.

For example, `config/boards/rock-5b.conf`

```
BOARDFAMILY="rockchip-rk3588"
OVERLAY_PREFIX="rock-5b"
```

In generated image, in file `/boot/armbianEnv.txt`

```
overlay_prefix=rockchip-rk3588
```

But it must be
```
overlay_prefix=rock-5b
```

### Branch

main (main development branch)

### On which host OS are you running the build script and observing this problem?

Ubuntu 24.04 Noble

### Are you building on Windows WSL2?

- [ ] Yes, my Ubuntu/Debian/OtherOS is running on WSL2

### Relevant log URL

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct