introduce new config symbol for experimental stuff

The new symbol will hide experimental stuff, so that
only known to work combinations are allowed to choose,
when ADK_EXPERIMENTAL disabled.

target/config/Config.in.adk

@@ -17,6 +17,13 @@ config ADK_DL_DIR
 	  Configure the download directory for all source packages.
 	  Use an absolute path.
 
+config ADK_EXPERIMENTAL
+	bool "Enable support for experimental features"
+	default n
+	help
+	  This enables combinations of toolchain components or kernel
+          and packages, which might be unsafe to use.
+
 config ADK_BUILD_WITH_DEBUG
 	bool "Build everything with debug information and allow debug packages"
 	default n

target/config/Config.in.kernelversion.choice

@@ -8,14 +8,20 @@ default ADK_KERNEL_VERSION_4_0_5
 
 config ADK_KERNEL_VERSION_GIT
 	bool "linux-git"
+	depends on (ADK_TARGET_SYSTEM_RASPBERRY_PI \
+		|| ADK_TARGET_SYSTEM_RASPBERRY_PI2) && ADK_EXPERIMENTAL
 	
 config ADK_KERNEL_VERSION_4_1_0_RC5
 	bool "4.1.0rc5"
 	select ADK_KERNEL_VERSION_4_1
+	depends on (ADK_TARGET_SYSTEM_RASPBERRY_PI \
+		|| ADK_TARGET_SYSTEM_RASPBERRY_PI2) && ADK_EXPERIMENTAL
 
 config ADK_KERNEL_VERSION_4_0_5
 	bool "4.0.5"
 	select ADK_KERNEL_VERSION_4_0
+	depends on (ADK_TARGET_SYSTEM_RASPBERRY_PI \
+		|| ADK_TARGET_SYSTEM_RASPBERRY_PI2) && ADK_EXPERIMENTAL
 
 config ADK_KERNEL_VERSION_3_18_14
 	bool "3.18.14"