Here is an exemple of the use of the maven build helper plugin to include an arbitrary folder as a source folder. This approach is compatible with eclipse as long as you have m2e installed with the build-helper connector.

build-helper-snippet.xml

  <build>
		<plugins>
			<plugin>
				<groupId>org.codehaus.mojo</groupId>
				<artifactId>build-helper-maven-plugin</artifactId>
				<version>1.7</version>
				<executions>
					<execution>
						<id>add-source</id>
						<phase>generate-sources</phase>
						<goals>
							<goal>add-source</goal>
						</goals>
						<configuration>
							<sources>
								<source>src/config</source>
							</sources>
						</configuration>
					</execution>
				</executions>
			</plugin>

		</plugins>
	</build>