linux: Release 4.12.8-openpower1

This brings the OpenPower kernel up to the latest stable kernel release.
4.12.8 contains an important fix for Power9 DD2:

  powerpc: Fix /proc/cpuinfo revision for POWER9 DD2

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/barreleye_defconfig b/openpower/configs/barreleye_defconfig
index 0e86241..675661d 100644
--- a/openpower/configs/barreleye_defconfig
+++ b/openpower/configs/barreleye_defconfig
@@ -16,7 +16,7 @@
 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="4.12.5"
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.12.8"
 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_defconfig"