kernel: Move to Linux 4.15.6-openpower1

This moves us to the 4.15 kernel series. The defconfig has been
refreshed, with the major difference being NVMe multipath support is now
enabled in the kernel.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/zz_defconfig b/openpower/configs/zz_defconfig
index f78a43f..42b1764 100644
--- a/openpower/configs/zz_defconfig
+++ b/openpower/configs/zz_defconfig
@@ -13,7 +13,7 @@
 BR2_ROOTFS_POST_BUILD_SCRIPT="../openpower/scripts/fixup-target-var ../openpower/scripts/firmware-whitelist"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_VERSION=y
-BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.14.20"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.15.6"
 BR2_LINUX_KERNEL_PATCH="$(BR2_EXTERNAL_OP_BUILD_PATH)/linux"
 BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
 BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="$(BR2_EXTERNAL_OP_BUILD_PATH)/configs/linux/skiroot_p9_defconfig"