buildroot: Move to 2017.08

All defconfigs are regenerated with the new buildroot version. GCC 6 and
Binutils 2.28 are now the default, so they are not specified in the
defconfig.

Updated packages (since 2017.05.1):

  busybox: 1.26.2 -> 1.27.1
  util-linux: 2.29.2 -> 2.30.1
  iprutils: 2.4.10.1 -> 2.4.14.1
  pciutils: 3.5.2 -> 3.5.5
  linux-firmware: 6d3bc8886517d171068fd1263176b8b5c51df204 -> a3a26af24e29c818ef9b5661856018e21a5c49fb
  lvm2: 2.02.170 -> 2.02.171
  nvme: v0.3 -> v1.3

Fixes: https://github.com/open-power/op-build/issues/1187
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 52777b0..3152edd 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2017.05-op-build
+	branch = 2017.08-op-build
 	url = https://github.com/open-power/buildroot