buildroot: Move to 2015.11.1 release

The November 2015 buildroot release incroperates more of our patches,
further reducing the delta between what we require for openpower and
upstream.

 - Update defconfigs.
   * The Chelsio T4 option BR2_PACKAGE_LINUX_FIRMWARE_CXGB4
     has been renamed to BR2_PACKAGE_LINUX_FIRMWARE_CXGB4_T4 to
     better account for the fact that a T5 variant exists.
   * Explictly set kernel version to the same as used for GCC headers,
     and set headers version to 4.2.6.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 2c4a18c..20509fa 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
+	branch = 2015.11.1-op-build
 	url = https://github.com/open-power/buildroot