Update to buildroot 2016.05

 - package/dropbear: Add BR2_PACKAGE_DROPBEAR_SERVER option
 - host-fakeroot: re-run autotools to fix build on ppc64le host
 - linux-firmware: Update bnx2x firmware version

Configuration changes:

 - BR2_BINUTILS_VERSION_2_26_X selects the most modern version
 - BR2_LINUX_KERNEL_CUSTOM_VERSION replaces LATEST_VERSION as upstream
   tracks a newer version than we are using
 - BR2_PACKAGE_SKIBOOT is default y if (BR2_OPENPOWER_PLATFORM && BR2_powerpc_power8)
   BR2_PACKAGE_POWERPC_UTILS is selected by petitboot
 - The BR2_TOOLCHAIN_BUILDROOT_LIBSTDCPP symbol was never upstreamed.
   It's implemented by BR2_INSTALL_LIBSTDCPP in upstream, and we don't
   have it enabled.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index 20509fa..6bda627 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2015.11.1-op-build
+	branch = 2016.05-op-build
 	url = https://github.com/open-power/buildroot