linux-ext: add fbtft hint for linux kernel >= v4.0

Since v4.0 the fbtft drivers are included in the linux kernel
staging area.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

linux/Config.ext.in

@@ -50,7 +50,9 @@ config BR2_LINUX_KERNEL_EXT_FBTFT
 	select BR2_PACKAGE_FBTFT
 	help
 	  Linux Framebuffer drivers for small TFT LCD display modules,
-	  e.g. Adafruit PiTFT displays for Raspberry Pi.
+	  e.g. Adafruit PiTFT displays for Raspberry Pi (this extra
+	  package is only needed for linux kernels until v3.19, since
+	  v4.0 the drivers are included in the staging area).
 
 	  To enable fbtft, e.g. for Adafruit 2.8 PiTFT, enable the
 	  following kernel configurations: