Restoring the option to ignore p2 mirrors via the Maven settings

Commit 7e6d595 has removed the option
to use the settings.xml to disable the usage of p2 mirrors.

See https://github.com/eclipse-tycho/tycho/wiki/Frequently-asked-questions#how-do-i-disable-p2-mirrors

Restores this option and give the command line option priority.

p2-maven-plugin/src/main/java/org/eclipse/tycho/p2maven/transport/RemoteArtifactRepositoryManagerAgentFactory.java

@@ -20,6 +20,7 @@
 import org.eclipse.equinox.p2.core.spi.IAgentServiceFactory;
 import org.eclipse.equinox.p2.repository.artifact.IArtifactRepositoryManager;
 import org.eclipse.tycho.IRepositoryIdManager;
+import org.eclipse.tycho.core.shared.MavenContext;
 
 @Component(role = IAgentServiceFactory.class, hint = "org.eclipse.equinox.p2.repository.artifact.IArtifactRepositoryManager")
 public class RemoteArtifactRepositoryManagerAgentFactory implements IAgentServiceFactory {
@@ -33,6 +34,9 @@ public class RemoteArtifactRepositoryManagerAgentFactory implements IAgentServic
 	@Requirement
 	MavenAuthenticator authenticator;
 
+	@Requirement
+	MavenContext mavenContext;
+
     @Override
     public Object createService(IProvisioningAgent agent) {
         IArtifactRepositoryManager plainRepoManager = (IArtifactRepositoryManager) new ArtifactRepositoryComponent()
@@ -48,6 +52,10 @@ private boolean getDisableP2MirrorsConfiguration() {
         String key = "tycho.disableP2Mirrors";
 		String value = System.getProperty(key);
 
+		if (value == null) {
+			value = mavenContext.getSessionProperties().getProperty(key);
+		}
+
         boolean disableP2Mirrors = Boolean.parseBoolean(value);
 		if (disableP2Mirrors && logger.isDebugEnabled()) {
             String message = key + "=" + value + " -> ignoring mirrors specified in p2 artifact repositories";