buildroot: Move to 2019.02 release

 http://lists.busybox.net/pipermail/buildroot/2019-March/244273.html
 https://git.buildroot.net/buildroot/plain/CHANGES?id=2019.02

Notable changes:

 - Require Python >= 2.7
 - openssl: Bump to 1.1.1x series, bringing TLSv1.3 support and long
   term support.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/.gitmodules b/.gitmodules
index e128ce3..5c83584 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,4 +1,4 @@
 [submodule "buildroot"]
 	path = buildroot
-	branch = 2018.11-op-build
+	branch = 2019.02-op-build
 	url = https://github.com/open-power/buildroot