docs: update autoupdater documentation

docs/features/autoupdater.rst

@@ -7,8 +7,11 @@ Building Images
 ---------------
 
 By default, the autoupdater is disabled (as it is usually not helpful to have unexpected updates
-during development), but it can be enabled by setting the variable GLUON_BRANCH when building
-to override the default branch set in the site configuration.
+during development), but it can be enabled by setting the variable ``GLUON_AUTOUPDATER_ENABLED`` to ``1`` when building.
+It is also possible to override the default branch during build using the variable ``GLUON_AUTOUPDATER_BRANCH``.
+
+If a default branch is set neither in *site.conf* nor via ``GLUON_AUTOUPDATER_BRANCH``, the default branch is
+implementation-defined. Currently, the branch with the first name in alphabetical order is chosen.
 
 A manifest file for the updater can be generated with `make manifest`. A signing script (using
 ``ecdsautils``) can be found in the `contrib` directory. When creating the manifest, the
@@ -32,15 +35,16 @@ Automated nightly builds
 
 A fully automated nightly build could use the following commands:
 
-::
+.. code-block:: sh
 
     git pull
-    (git -C site pull)
+    # git -C site pull
     make update
     make clean GLUON_TARGET=ar71xx-generic
     NUM_CORES_PLUS_ONE=$(expr $(nproc) + 1)
-    make -j$NUM_CORES_PLUS_ONE GLUON_TARGET=ar71xx-generic GLUON_BRANCH=experimental GLUON_RELEASE=$GLUON_RELEASE
-    make manifest GLUON_BRANCH=experimental GLUON_RELEASE=$GLUON_RELEASE
+    make -j$NUM_CORES_PLUS_ONE GLUON_TARGET=ar71xx-generic GLUON_RELEASE=$GLUON_RELEASE \
+        GLUON_AUTOUPDATER_BRANCH=experimental GLUON_AUTOUPDATER_ENABLED=1
+    make manifest GLUON_RELEASE=$GLUON_RELEASE GLUON_AUTOUPDATER_BRANCH=experimental
     contrib/sign.sh $SECRETKEY output/images/sysupgrade/experimental.manifest
 
     rm -rf /where/to/put/this/experimental

docs/site-example/site.conf

@@ -164,7 +164,8 @@
   },
 
   autoupdater = {
-    -- Default branch. Don't forget to set GLUON_BRANCH when building!
+    -- Default branch (optional), can be overridden by setting GLUON_AUTOUPDATER_BRANCH when building.
+    -- Set GLUON_AUTOUPDATER_ENABLED to enable the autoupdater by default for newly installed nodes.
     branch = 'stable',
 
     -- List of branches. You may define multiple branches.

docs/user/getting_started.rst

@@ -171,10 +171,12 @@ usually be set on the command line or in ``site.mk``.
 Common variables
 ................
 
-GLUON_BRANCH
-  Sets the default branch of the autoupdater. If unset, the autoupdater is disabled
-  by default. For the ``make manifest`` command, GLUON_BRANCH defines the branch to
-  generate a manifest for.
+GLUON_AUTOUPDATER_BRANCH
+  Overrides the default branch of the autoupdater set in ``site.conf``. For the ``make manifest`` command,
+  ``GLUON_AUTOUPDATER_BRANCH`` defines the branch to generate a manifest for.
+
+GLUON_AUTOUPDATER_ENABLED
+  Set to ``1`` to enable the autoupdater by default for newly installed nodes.
 
 GLUON_DEPRECATED
   Controls whether images for deprecated devices should be built. The following
@@ -225,7 +227,7 @@ GLUON_DEBUG
   similar tools. Requires a device or target with at least 16 MB of flash space, e.g. `x86-64`. Unset by default.
 
 GLUON_MINIFY
-  Setting ``GLUON_MINIFY=0`` will omit the minification of scripts during the build process. By 
+  Setting ``GLUON_MINIFY=0`` will omit the minification of scripts during the build process. By
   default the flag is set to ``1``. Disabling the flag is handy if human readable scripts on the
   devices are desired for development purposes. Be aware that this will increase the size of the
   resulting images and is therefore not suitable for devices with small flash chips.

docs/user/site.rst

@@ -418,12 +418,16 @@ poe_passthrough \: optional
 autoupdater \: package
     Configuration for the autoupdater feature of Gluon.
 
+    Specifying a default branch in *site.conf* is optional. See
+    :doc:`../features/autoupdater` for information how to change the behaviour
+    of the autoupdater during image build.
+
     The mirrors are checked in random order until the manifest could be downloaded
     successfully or all mirrors have been tried.
     ::
 
       autoupdater = {
-        branch = 'stable',
+        branch = 'stable', -- optional
         branches = {
           stable = {
             name = 'stable',