rainier_defconfig: use p10 Kernel

Need to use the (non-upstream) p10 Linux Kernel for the time being.

This requires us to use a non-upstream buildroot as well.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
diff --git a/.gitmodules b/.gitmodules
index 085527d..5cda80d 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2019.05-op-build
-	url = https://github.com/open-power/buildroot
+	branch = master-p10
+	url = git@github.ibm.com:klausk/buildroot.git