buildroot: Move to 2018.05 release

 https://git.buildroot.net/buildroot/plain/CHANGES?id=2018.05
 http://lists.busybox.net/pipermail/buildroot/2018-June/222697.html

Highlights for OpenPower Firmware:

 - Git caching: you don't need to re-clone from scratch when moving to a
 different version of a package

Updated packages:

  busybox: 1.27.2 -> 1.28.4
  glibc: glibc-2.26-146 -> glibc-2.27-57
  dropbear: 2017.75 -> 2018.76
  ethtool: 4.13 -> 4.16
  ncurses: 6.0 -> 6.1
  util-linux: 2.31.1 -> 2.32

Updated host packages:

  lz4: v1.7.5 -> v1.8.2
  ncurses: 6.0 -> 6.1
  gcc-initial: 6.4.0 -> 7.3.0
  gcc-final: 6.4.0 -> 7.3.0
  lzip: 1.19 -> 1.20
  gawk: 4.1.4 -> 4.2.1
  libopenssl: 1.0.2n -> 1.0.2o
  squashfs: 3de1687d7432ea9b302c2db9521996f506c140a3 -> e38956b92f738518c29734399629e7cdb33072d3

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index d756b37..65c4f27 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2018.02-op-build
+	branch = 2018.05-op-build
 	url = https://github.com/open-power/buildroot