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/linux/skiroot_p9_defconfig b/openpower/configs/linux/skiroot_p9_defconfig
index 7b08bae..309f182 100644
--- a/openpower/configs/linux/skiroot_p9_defconfig
+++ b/openpower/configs/linux/skiroot_p9_defconfig
@@ -13,6 +13,7 @@
 CONFIG_TASK_DELAY_ACCT=y
 CONFIG_TASK_XACCT=y
 CONFIG_TASK_IO_ACCOUNTING=y
+# CONFIG_CPU_ISOLATION is not set
 CONFIG_IKCONFIG=y
 CONFIG_IKCONFIG_PROC=y
 CONFIG_LOG_BUF_SHIFT=20
@@ -75,6 +76,7 @@
 CONFIG_BLK_DEV_RAM_SIZE=65536
 CONFIG_VIRTIO_BLK=m
 CONFIG_BLK_DEV_NVME=m
+CONFIG_NVME_MULTIPATH=y
 CONFIG_EEPROM_AT24=y
 # CONFIG_CXL is not set
 CONFIG_BLK_DEV_SD=m
@@ -135,6 +137,7 @@
 CONFIG_IXGBE=m
 CONFIG_I40E=m
 CONFIG_MLX4_EN=m
+# CONFIG_MLX4_CORE_GEN2 is not set
 CONFIG_MLX5_CORE=m
 CONFIG_MLX5_CORE_EN=y
 # CONFIG_MLX5_MPFS is not set
@@ -152,6 +155,7 @@
 CONFIG_SERIAL_8250=y
 CONFIG_SERIAL_8250_CONSOLE=y
 CONFIG_IPMI_HANDLER=y
+# CONFIG_IPMI_PROC_INTERFACE is not set
 CONFIG_IPMI_DEVICE_INTERFACE=y
 CONFIG_IPMI_POWERNV=y
 CONFIG_HW_RANDOM=y
@@ -159,7 +163,6 @@
 # CONFIG_I2C_COMPAT is not set
 CONFIG_I2C_CHARDEV=y
 # CONFIG_I2C_HELPER_AUTO is not set
-# CONFIG_RC_CORE is not set
 CONFIG_DRM=y
 CONFIG_DRM_RADEON=y
 CONFIG_DRM_AST=m