linux: use BR2_LINUX_KERNEL_LATEST_VERSION as option name

Use a option name that doesn't change for every new 3.x version.
It's easier to maintain and consistent with barebox.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

linux/Config.in

@@ -20,7 +20,7 @@ if BR2_LINUX_KERNEL
 choice
 	prompt "Kernel version"
 
-config BR2_LINUX_KERNEL_3_9
+config BR2_LINUX_KERNEL_LATEST_VERSION
 	bool "3.9"
 
 config BR2_LINUX_KERNEL_SAME_AS_HEADERS
@@ -76,7 +76,7 @@ config BR2_LINUX_KERNEL_CUSTOM_GIT_VERSION
 
 config BR2_LINUX_KERNEL_VERSION
 	string
-	default "3.9" if BR2_LINUX_KERNEL_3_9
+	default "3.9" if BR2_LINUX_KERNEL_LATEST_VERSION
 	default BR2_DEFAULT_KERNEL_HEADERS if BR2_LINUX_KERNEL_SAME_AS_HEADERS
 	default BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE if BR2_LINUX_KERNEL_CUSTOM_VERSION
 	default "custom" if BR2_LINUX_KERNEL_CUSTOM_TARBALL