build: Refactor image configuration methods

Fixes a former lack of correct environment variable and build argument substitution on image configuration fields.

Instead of providing one general ConfigureImage method on a build.Target, provide an exported field Image and new corresponding struct type that provides builder methods for altering specific parts of the target's internal image configuration. This makes the interface clearer and ensures that environment variable and build argument substitution on new values is performed.

Change-Id: I041f7c55ea09899b33773c1872e9543422a99e36