[Bug]: rk3528: edge U-Boot cached artifact has empty device tree (0-byte FDT in u-boot.itb)

### What happened?

## Summary

The ORAS-cached U-Boot artifact for RK3528 `edge` contains a `u-boot.itb` with a **0-byte FDT**, causing all RK3528 edge builds to fail at U-Boot with:

```
No valid device tree binary found - please append one to U-Boot binary
ERROR ### Please RESET the board
```

The `current` variant works correctly with a valid 7992-byte FDT.

## Affected artifact
`ghcr.io/armbian/os/uboot-nanopi-zero2-edge:2017.09-S8e7d-Pb178-Hedea-V2226-B61c5-R448a`

## Analysis
Using `dumpimage -l` on the `u-boot.itb` from each variant:

| | current | edge |
|--|---------|------|
| **FDT Data Size** | **7992 Bytes** | **0 Bytes** |
| **FDT SHA256** | `db83a073...` | `e3b0c442...` (SHA256 of empty data) |
| **Boot result** | Works | Fails - no DTB |

The ATF blobs (atf-1, atf-2, atf-3) are identical between both variants. Only the U-Boot binary and FDT differ.

## Reproduction

```
bash
# Build edge image for any RK3528 board
./compile.sh BOARD=nanopi-zero2 BRANCH=edge RELEASE=noble BUILD_MINIMAL=yes BUILD_DESKTOP=no KERNEL_CONFIGURE=no

```

The board fails to boot with the error above.

## Additional finding: local build also affected

Building with ARTIFACT_IGNORE_CACHE=yes correctly compiles U-Boot from source. The build tree contains a valid 7992-byte u-boot.dtb and a correct u-boot.itb. These get packaged into a correct deb in output/packages-hashed/. However, the reversioned deb that ends up in output/debs/ still contains the old broken artifact with the 0-byte FDT.

## Workaround
After building with ARTIFACT_IGNORE_CACHE=yes, manually copy the correct deb:

```
cp output/packages-hashed/global/linux-u-boot-nanopi-zero2-edge_*.deb \
output/debs/linux-u-boot-nanopi-zero2-edge_26.02.0-trunk_arm64__*.deb

```Then rebuild the image.

## Impact
Currently only the NanoPi Zero2 has edge in its KERNEL_TARGET for RK3528, but this would block any other RK3528 board from enabling edge kernel support.

Discovered while working on [#9500](https://github.com/armbian/build/pull/9500).

### How to reproduce?

```
# Build edge image for any RK3528 board
./compile.sh BOARD=nanopi-zero2 BRANCH=edge RELEASE=noble BUILD_MINIMAL=yes BUILD_DESKTOP=no KERNEL_CONFIGURE=no

```
Then try to boot the image on a Nanopi Zero2

### 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?

- [x] 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