fix autoupdater not being 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 rewrite of the build.sh, as the variable is not set by it anymore 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