fix upgrade issues to future gluon releases

v2017 and later have some changes which causes config loss
when upgraded directly.

This release basically aims to keep the config for a future release
of a current gluon (which then has a better hardware support)

CHANGELOG.md

@@ -2,11 +2,10 @@
 
 ## v2018.0
 
-UNRELEASED
-
- - Updated to Gluon v2016.2.7-2-g3d824bd2
+ - Updated to Gluon v2016.2.7-3-g35c9f9ae
    - ar71xx: fix MAC addresses on TP-Link TL-WR1043ND v4
    - modules: update url to chaos calmer git repository after upstream move
+   - ar71xx: sysupgrade: improve CPE/WBS 210/510 validation
 
  - Makefile
    - Enabled firmware with ath10k WLAN driver

Makefile

@@ -1,6 +1,6 @@
 GLUON_BUILD_DIR := gluon-build
 GLUON_GIT_URL := https://github.com/freifunk-gluon/gluon.git
-GLUON_GIT_REF := 3d824bd21d7f6d5db6dc238d2a68b62de471aa2a
+GLUON_GIT_REF := 35c9f9aecc5de53324ee61904faa1c5763d61134  # latest v2016.2.x
 
 SECRET_KEY_FILE ?= ${HOME}/.gluon-secret-key