linux: bump default to version 4.15.6

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

linux/Config.in

@@ -116,7 +116,7 @@ endif
 
 config BR2_LINUX_KERNEL_VERSION
 	string
-	default "4.15.5" if BR2_LINUX_KERNEL_LATEST_VERSION
+	default "4.15.6" if BR2_LINUX_KERNEL_LATEST_VERSION
 	default "v4.4.112-cip18" if BR2_LINUX_KERNEL_LATEST_CIP_VERSION
 	default BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE \
 		if BR2_LINUX_KERNEL_CUSTOM_VERSION