Support relaxed binding for properties before environment is prepared

Fixes gh-17843

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -338,6 +338,7 @@ public class SpringApplication {
 		// Create and configure the environment
 		ConfigurableEnvironment environment = getOrCreateEnvironment();
 		configureEnvironment(environment, applicationArguments.getSourceArgs());
+		ConfigurationPropertySources.attach(environment);
 		listeners.environmentPrepared(environment);
 		bindToSpringApplication(environment);
 		if (!this.isCustomEnvironment) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationTests.java

@@ -1072,6 +1072,14 @@ public class SpringApplicationTests {
 				.getBean("someBean")).isEqualTo("override");
 	}
 
+	@Test
+	public void relaxedBindingShouldWorkBeforeEnvironmentIsPrepared() {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		application.setWebApplicationType(WebApplicationType.NONE);
+		this.context = application.run("--spring.config.additionalLocation=classpath:custom-config/");
+		assertThat(this.context.getEnvironment().getProperty("hello")).isEqualTo("world");
+	}
+
 	private Condition<ConfigurableEnvironment> matchingPropertySource(final Class<?> propertySourceClass,
 			final String name) {
 		return new Condition<ConfigurableEnvironment>("has property source") {

spring-boot-project/spring-boot/src/test/resources/custom-config/application.yml

@@ -0,0 +1 @@
+hello: world