debian/README.source: Rewrite and expand explanation of kernel config file construction

debian/README.source

@@ -132,16 +132,37 @@ architectures amd64, i386 and x32.
 Kernel config files
 ===================
 
-Configuration files are constructed dynamically from a number of config
-files under debian/config, in the following order:
+Each kernel configuration file is constructed dynamically from a
+number of files under debian/config.  They are read in the following
+order, such that files later on the list can override settings from
+earlier files.  Most of the files are optional and the filenames can
+generally be overridden by explicit lists (possibly empty) specified
+in the 'defines' files.
 
-- config
-- kernelarch-%kernel-arch/config
-- %arch/config
-- %arch/config.%flavour
-- featureset-%featureset/config
-- %arch/%featureset/config
-- %arch/%featureset/config.%flavour
+1. Common:
+   - Default filename: config
+   - Filename list: [image]configs in defines
+2. Per kernel architecture:
+   - Filename: kernelarch-<karch>/config (optional)
+3. Per architecture:
+   - Default filename: <arch>/config
+   - Filename list: [image]configs in <arch>/defines
+4. Per architecture and flavour:
+   - Default filename: <arch>/config.<flavour> (optional)
+   - Filename list: [<flavour>_image]configs in <arch>/defines
+5. Per featureset:
+   - Default filename: featureset-<fset>/config (optional)
+   - Filename list: [image]configs in featureset-<fset>/defines
+6. Per architecture and featureset:
+   - Default filename: <arch>/<fset>/config (optional)
+   - Filename list: [image]configs in <arch>/<fset>/defines
+7. Per architecture, featureset, and flavour:
+   - Default filename: <arch>/<fset>/config.<flavour> (optional)
+   - Filename list: [<flavour>_image]configs in <arch>/<fset>/defines
+
+You can check the final list of configuration files by reading
+debian/rules.gen.  Each binary-arch_<arch>_<fset>_<flavour>_real
+rule passes the list to debian/rules.real as the KCONFIG variable.
 
 Control file
 ============

debian/changelog

@@ -6,6 +6,8 @@ linux (4.6-1~exp2) UNRELEASED; urgency=medium
     directory (Closes: #823632; thanks to Helmut Grohne)
   * [rt] Update to 4.6-rc7-rt1 and re-enable
   * Fold debian/config/README into debian/README.source
+  * debian/README.source: Rewrite and expand explanation of kernel config file
+    construction
 
  -- Ben Hutchings <ben@decadent.org.uk>  Tue, 17 May 2016 23:33:30 +0100