images: remove unused config_file field; fix README show-image example

The config_file field on ImageConfig is no longer read; full_yaml_path()
derives the YAML from config_path()/output_and_config. Left in place it
made show-image <name> config-file return a stale default. Remove the
field and update the README to show the actually-used show-image field
(base-image, as the build pipeline invokes it).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: bfjelds

tests/images/builder/README.md

@@ -100,7 +100,7 @@ The following commands are supported:
     python3 ./testimages.py dependencies <image_name>
     # Build an image locally
     python3 ./testimages.py build <image_name>
-    # Show key info about the image, such as name, source, config path, etc.
+    # Show key info about the image, such as name, source, base image, etc.
     python3 ./testimages.py show-image <image_name> <field>
     # Show info on key artifacts, such as the Image Customizer version or container image
     python3 ./testimages.py show-artifact
@@ -117,10 +117,10 @@ To build an image with Builder or test your changes to Builder, follow these ste
     ./testimages.py download-image <base_image_name>
 ```
 
-1. If necessary, update the Image Customizer config for the image you want to build, by modifying the corresponding YAML in `test-images/platform-integration-images`. You can also find the relevant YAML by running:
+1. If necessary, update the Image Customizer config for the image you want to build, by modifying the corresponding YAML in `test-images/platform-integration-images`. You can inspect fields of an image config — for example, the base image it builds on, as the build pipeline does — by running:
 
 ```bash
-    python3 ./testimages.py show-image <image_name> config-file
+    python3 ./testimages.py show-image <image_name> base-image
 ```
 
 1. Build the image:

tests/images/builder/__init__.py

@@ -168,9 +168,6 @@ class ImageConfig:
     # Second-level dir, generally same as name
     config: str = None
 
-    # YAML config file inside the config dir
-    config_file: Path = Path("base/baseimg.yaml")
-
     # The base image to use
     base_image: BaseImage = BaseImage.BAREMETAL