config: add big EXPERIMENTAL option

As discussed in the today's (2020-12-10) meeting, add a new option to
menuconfig to group the selection of all experimental features to be
selected by default.
Developers are recommended to make use of this new symbol to guard
new features.
Other developers and community members should feel encouraged to
build with this flag enabled to help testing and provide feedback.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

config/Config-build.in

@@ -5,6 +5,20 @@
 # See /LICENSE for more information.
 #
 
+config EXPERIMENTAL
+	bool "Enable experimental features by default"
+	default n
+	help
+	  Set this option to build with latest bleeding edge features
+	  which may or may not work as expected.
+	  If you would like to help the development of OpenWrt, you are
+	  encouraged to set this option and provide feedback (both
+	  positive and negative). But do so only if you know how to
+	  recover your device in case of flashing potentially non-working
+	  firmware.
+	
+	  If you plan to use this build in production, say NO!
+
 menu "Global build settings"
 
 	config JSON_OVERVIEW_IMAGE_INFO
@@ -51,7 +65,7 @@ menu "Global build settings"
 	config TESTING_KERNEL
 		bool "Use the testing kernel version"
 		depends on HAS_TESTING_KERNEL
-		default n
+		default EXPERIMENTAL
 		help
 		  If the target supports a newer kernel version than the default,
 		  you can use this config option to enable it