fix autoupdater not enabled by default

until Gluon 2017.1 the GLUON_BRANCH variable in the site.mk must explicitly be exported or it will have no effect
this issue exist since the build.sh rewrite, which previously set it
during a build

site.mk

@@ -33,7 +33,7 @@ GLUON_SITE_PACKAGES := \
 
 # Allow overriding the these variables from the command line
 GLUON_RELEASE ?= $(patsubst v%,%,$(shell git -C $(GLUON_SITEDIR) describe --tags --dirty=+))
-GLUON_BRANCH ?= stable
+export GLUON_BRANCH ?= stable
 GLUON_PRIORITY ?= 0
 GLUON_LANGS ?= en de
 GLUON_REGION ?= eu