load gradle build variables or properties from properties file or system environment

build.gradle

apply from: "envutils.gradle"

// loading to data map
def data = loadFromEnvOrProp()

buildTypes {

        release {
            minifyEnabled false
            buildConfigField "String", "API_URL", data["api.url"]
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
}

envutils.gradle

class Environment {

    def data

    def getProperty(String name){

        if(data[name]){
            return data[name]
        }

        return System.getenv(name)
    }
}

ext.loadFromEnvOrProp = { prop_file ->

    def data = [:]

    if(prop_file == null){
        prop_file = "gradle.properties"
    }

    if (project.rootProject.file(prop_file).exists()) {

        Properties properties = new Properties()
        properties.load(project.rootProject.file(prop_file).newDataInputStream())
        properties.propertyNames().each { name ->

            data[name] = properties.getProperty(name)
        }

    }

    return new Environment(data:data)
}