buildroot: Move to 2018.11.3 stable release

Upstream release notes:

 http://lists.busybox.net/pipermail/buildroot/2019-February/243960.html
 https://git.buildroot.net/buildroot/plain/CHANGES?id=2018.11.3

This includes a patch to move to 4.20 as the default kernel. As we're
still on 4.19, we must now set the buildroot kernel series option.

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/configs/pseries_defconfig b/openpower/configs/pseries_defconfig
index e18d14a..cfd3084 100644
--- a/openpower/configs/pseries_defconfig
+++ b/openpower/configs/pseries_defconfig
@@ -1,5 +1,6 @@
 BR2_powerpc64le=y
 BR2_powerpc_power8=y
+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_4_19=y
 BR2_BINUTILS_EXTRA_CONFIG_OPTIONS="--enable-targets=powerpc64-linux"
 BR2_GCC_VERSION_6_X=y
 BR2_EXTRA_GCC_CONFIG_OPTIONS="--enable-targets=powerpc64-linux --disable-libsanitizer"