Error out if rootfs type is not layers
Error out if rootfs type is not layers

The image spec requries that
````
MUST be set to layers. Implementations MUST generate an error if
they encounter a unknown value while verifying or unpacking an image.
```
The image-tool should act as the image spec says.

Signed-off-by: Lei Jiang <9ac444d2b5df3db1f31aa1c6462ac8e9e2bde241@huawei.com>

xiekeyang/image-tools,opencontainers/image-tools,xiekeyang/image-tools,opencontainers/image-tools