How do I read properties defined in local.properties in build.gradle

You can do that in this way:

Properties properties = new Properties()
properties.load(project.rootProject.file('local.properties').newDataInputStream())
def sdkDir = properties.getProperty('sdk.dir')
def ndkDir = properties.getProperty('ndk.dir')

Use project.rootProject if you are reading the properties file in a sub-project build.gradle:

.
├── app
│   ├── build.gradle <-- You are reading the local.properties in this gradle build file
│   └── src
├── build.gradle
├── gradle
├── gradlew
├── gradlew.bat
├── settings.gradle
└── local.properties

In case the properties file is in the same sub-project directory you can use just project.