buildroot: Move to 2017.05 release

The following packages have been updated since buildroot 2017.02.2:

 lvm2: 2.02.168 -> 2.02.170
 check: 0.10.0 -> 0.11.0
 kmod: 23 -> 24
 powerpc-utils: v1.3.1 -> v1.3.3
 glibc: 2.24 -> 2.25
 dropbear: 2016.74 -> 2017.75
 ethtool: 4.8 -> 4.10
 ncurses: 5.9 -> 6.0
 eudev: 3.2.1 -> 3.2.2
 dtc: 1.4.1 -> 1.4.4
 elfutils: 0.166 -> 0.169

In addition, merge the 2017.05.x stable branch to bring in a build fix
for kmod.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 324fa00..52777b0 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2017.02-op-build
+	branch = 2017.05-op-build
 	url = https://github.com/open-power/buildroot