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/overlay/etc/locale b/openpower/overlay/etc/locale
new file mode 100644
index 0000000..6556836
--- /dev/null
+++ b/openpower/overlay/etc/locale
@@ -0,0 +1 @@
+LANG=en_US.utf-8