gradle properties in Android Studio

build.gradle

// Other build stuff
 task ndkBuild(type:Exec) {

    if(androidNdkPath != null) {
        def ndkBuild = new File(androidNdkPath, 'ndk-build')
        commandLine ndkBuild
    } else {
        // do something else
    }
}

gradle.properties

# file goes in ~/.gradle/gradle.properties
androidNdkPath=/path/to/your/ndk

updated.build.gradle

// Other build stuff
 task ndkBuild(type:Exec) {

    if (System.env.ANDROID_NDK_HOME != null) {
        def ndkBuild = new File(System.env.ANDROID_NDK_HOME, 'ndk-build')
        commandLine ndkBuild
    } else if(androidNdkPath != null) {
        def ndkBuild = new File(androidNdkPath, 'ndk-build')
        commandLine ndkBuild
    } else {
        commandLine 'ndk-build'
    }
}