Move overlay from buildroot into openpower/

We'd like to make some customisations to the overlay, without adding more
delta to the openpower buildroot tree.

This change updates to a buildroot tree that no longer carries the
overlay/ directory, and adds the overlay (and necessary config updates)
to our own op-build tree.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/openpower/configs/habanero_defconfig b/openpower/configs/habanero_defconfig
index 0d83ffa..9bcec45 100644
--- a/openpower/configs/habanero_defconfig
+++ b/openpower/configs/habanero_defconfig
@@ -39,7 +39,7 @@
 BR2_GENERATE_LOCALE="en_US.UTF-8"
 BR2_ROOTFS_DEVICE_CREATION_DYNAMIC_EUDEV=y
 BR2_TARGET_GENERIC_GETTY_PORT="hvc0"
-BR2_ROOTFS_OVERLAY="overlay"
+BR2_ROOTFS_OVERLAY="../openpower/overlay"
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_VERSION="4.2"
 BR2_KERNEL_HEADERS_4_2=y