spring-boot-configuration-processor is an annotation processor that generates metadata about classes in your application that are annotated with @ConfigurationProperties. This metadata is used by your IDE (Eclipse, IntelliJ, or NetBeans) to provide auto-completion and documentation for the properties when editing application.properties and application.yaml files. You can learn a bit more about it in the relevant section of Spring Boot’s reference documentation.
Since Spring Boot 1.5.10, the exclusion is no longer necessary as com.vaadin.external.google:android-json is no longer a dependency of spring-boot-configuration-processor.