samples: add binderfs sample program

This adds a simple sample program mounting binderfs and adding, then
removing a binder device.  Hopefully, it will be helpful to users who want
to know how binderfs is supposed to be used.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

samples/Kconfig

@@ -147,6 +147,13 @@ config SAMPLE_VFIO_MDEV_MBOCHS
 	  Specifically it does *not* include any legacy vga stuff.
 	  Device looks a lot like "qemu -device secondary-vga".
 
+config SAMPLE_ANDROID_BINDERFS
+	bool "Build Android binderfs example"
+	depends on CONFIG_ANDROID_BINDERFS
+	help
+	  Builds a sample program to illustrate the use of the Android binderfs
+	  filesystem.
+
 config SAMPLE_STATX
 	bool "Build example extended-stat using code"
 	depends on BROKEN