doc: add a new template variable

Signify if the documentation is for a release or if it is the
development version from master.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>
diff --git a/doc/conf.py b/doc/conf.py
index 7db30f8..b4e5f32 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -158,8 +158,10 @@
 
 
 if tags.has('release'):
+    is_release = True
     docs_title = 'Docs / %s' %(version)
 else:
+    is_release = False
     docs_title = 'Docs'
 
 # The name for this set of Sphinx documents.  If None, it defaults to
@@ -344,6 +346,7 @@
 html_context = {
     'show_license': html_show_license,
     'docs_title': docs_title,
+    'is_release': is_release,
 }
 
 extlinks = {'jira': ('https://jira.zephyrproject.org/browse/%s', '')}