support/kconfig: Add missing target to README.buildroot

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

support/kconfig/README.buildroot

@@ -15,6 +15,7 @@ Then verify the toplevel targets work:
 	config
 	defconfig
 	menuconfig
+	nconfig
 	xconfig
 	gconfig
 	oldconfig