docs/build: add a short summary for some scripts

This summaries giving an overview of a scripts function and a short summary how
it's doing this.
Only the scripts are covered, that are used by the Freifunk-Berlin firmwarebuiler too.

docs/dev/build.rst

@@ -32,3 +32,47 @@ GLUON_BASE_FEEDS
     verbatim. By default, this contains a reference to the Gluon base packages;
     when using the Gluon build system to build a non-Gluon system, the variable
     can be set to the empty string.
+
+Helper scripts
+--------------
+
+Several tasks of the buildprocess have been separated from the Makefile into 
+external scripts, which are stored in the *scripts* directory. This was done to
+ease maintenance of these scripts and the Makefile, by avoiding a lot of escaping.
+These scripts are eighter bash or lua scripts that run on the build-system.
+
+default_feeds.sh
+    Defines the CONSTANT *DEFAULT_FEEDS* with the names of all feeds listed in 
+    *openwrt/feeds.conf.default*
+
+feeds.sh
+    Creates the *openwrt/feeds.conf* file from *FEEDS* and *DEFAULT_FEEDS*. The
+    feeds from *FEEDS* are linked to the matching subfolder of *packages/* and not
+    explicitly defined feeds of *DEFAULT_FEEDS* are setup as dummy (src-dummy).
+    This *openwrt/feeds.conf* is used to reinstall all packages of all feeds with
+    the *openwrt/scripts/feeds* tool.
+
+modules.sh
+    Defines the CONSTANT *GLUON_MODULES* and *FEEDS* by reading the *modules*
+    file of the root-folder and the site-folder. The returned variables look
+    like:
+
+    - *FEEDS*: "*feedA feedB ...*"
+    - *GLUON_MODULES*: "*openwrt packages/feedA packages/feedB ...*"
+
+patch.sh
+    (Re-)applies the patches from the *patches* directory to all *GLUON_MODULES*.
+    This is done for each repo by:
+
+    - create a temporary clone of the downloaded repo from the *packages* folder
+      - use only branch *base*
+    - apply all patches via *git am* on top of the *base* branch
+      - name this branch *patched*
+    - copy the temporary clone to the *packages* folder
+    - update all git submodules
+
+update.sh
+    Sets up a working clone of the *GLUON_MODULES* (external repos) from the external
+    source and installs it into *packages/* directory. It simply tries to set the *base*
+    branch of the cloned repo to the correct commit. If this fails it fetches the 
+    upstream branch and tries again to set the local *base* branch.