buildroot: Move to buildroot 2017.11

Upstream changelog:

  https://git.buildroot.net/buildroot/plain/CHANGES?id=2017.11

Changed packages from previous release (2017.08.2):

  mdadm: 3.3.4 -> 4.0
  glibc: 2.25 -> glibc-2.26-73-g4b692dffb95ac4812b161eb6a16113d7e824982e
  ethtool: 4.10 -> 4.13
  linux-headers: 4.14 -> 4.14.2
  util-linux: 2.30.1 -> 2.31
  eudev: 3.2.2 -> 3.2.4

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 3152edd..81d08d0 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2017.08-op-build
+	branch = 2017.11-op-build
 	url = https://github.com/open-power/buildroot