NDK can't find the application directory

You can use the following command

ndk-build -C your_project_path

Hope this will answer your question.

NDK_PROJECT_PATH is an environment variable so you don't have to include in the Android.mk file. Is nkd-build launched in the project directory?

For more info read the docs in docs/HOWTO.html in the NDK folder where I read

Starting with NDK r4, you can simply place the file under $PROJECT/jni/ and launch the 'ndk-build' script from your project tree.

If you want to use 'ndk-build' but place the file to a different location, use a GNU Make variable override as:

ndk-build NDK_APPLICATION_MK=/path/to/your/Application.mk