linux: bump default to version 4.17.15

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/Config.in

@@ -120,7 +120,7 @@ endif
 
 config BR2_LINUX_KERNEL_VERSION
 	string
-	default "4.17.14" if BR2_LINUX_KERNEL_LATEST_VERSION
+	default "4.17.15" if BR2_LINUX_KERNEL_LATEST_VERSION
 	default "v4.4.138-cip25" if BR2_LINUX_KERNEL_LATEST_CIP_VERSION
 	default BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE \
 		if BR2_LINUX_KERNEL_CUSTOM_VERSION