support a new build type for components with no artifacts

[massi-ang]

Describe the bug
I have a component with no artifacts, just a recipe. When I use gdk component publish, the command fails saying

[Errno 2] No such file or directory: '/Users/xxx/Projects/comp/greengrass-build/recipes/recipe.yaml'

To Reproduce
Steps to reproduce the behavior. If possible, provide a minimal amount of code that causes the bug.

Create a compoent with only a recipe, no artifacts. In the gdk-config.json file, provide a noop custom build command (eg date) so the gdk component build succeeds.

Call gdk component publish

Expected behavior
A clear and concise description of what you expected to happen.

Component is successfully published

Actual behavior
Tell us what actually happened.

Error

[Errno 2] No such file or directory: '/Users/xxx/Projects/comp/greengrass-build/recipes/recipe.yaml'

Environment

• OS: [e.g. Ubuntu 20.04] (If possible, provide output of uname -a)
• JDK version:
• Nucleus version:

Additional context
Add any other context about the problem here.

E.g. what is the impact of the bug?

[saranyailla]

Hi,
This is the expected behavior and thank you for reporting this use case.
If the customer chooses custom build in the config file, we expect customers to modify/build the recipes and artifacts as needed and have them available in the greengrass-build folder by the end of the build command execution.

These will later be used by the publish command.

I'm going to re-label this as documentation as it is needs to be specified in the documentation explicitly.

Edit:
Since, having only recipes with no artifacts for a component is a common use case, we decided to actively track the support for it.

[saranyailla]

Hi,
AWS Public Documentation for GDK CLI is updated with more details related to custom build command.