gradle: make release key optional

build.gradle

@@ -73,15 +73,6 @@ android {
 		targetCompatibility JavaVersion.VERSION_1_8
 	}
 
-	signingConfigs {
-		release {
-			storeFile file(RELEASE_STORE_FILE)
-			storePassword RELEASE_STORE_PASSWORD
-			keyAlias RELEASE_KEY_ALIAS
-			keyPassword RELEASE_KEY_PASSWORD
-		}
-	}
-
 	buildTypes {
 		debug {
 			minifyEnabled true
@@ -90,10 +81,24 @@ android {
 		release {
 			minifyEnabled true
 			proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard.cfg'
-			signingConfig signingConfigs.release
 		}
 	}
 
+	if (project.hasProperty("RELEASE_KEY_ALIAS")) {
+		signingConfigs {
+			release {
+				storeFile file(RELEASE_STORE_FILE)
+				storePassword RELEASE_STORE_PASSWORD
+				keyAlias RELEASE_KEY_ALIAS
+				keyPassword RELEASE_KEY_PASSWORD
+			}
+		}
+		buildTypes {
+			release {
+				signingConfig signingConfigs.release
+			}
+		}
+	}
 	sourceSets {
 		main {
 			manifest.srcFile 'AndroidManifest.xml'