doc: add section Factory Defaults to branding document

[skip ci]

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/branding.md

@@ -41,6 +41,62 @@ Verify the result after a build by inspecting:
     printed on a label on the device.
 
 
+Factory Defaults
+----------------
+
+The Infix default configuration, factory-config, is part static files
+and part per-device generated files, e.g., SSH hostkey and hostname.
+The latter is constructed from the file `/etc/hostname`, appended with
+the last three octets of the system's base MAC address.  To override the
+base hostname, set `BR2_TARGET_GENERIC_HOSTNAME` in your defconfig.
+
+The static files are installed by Infix `confd` in `/usr/share/confd/`
+at build time.  It contains two subdirectories:
+
+    /usr/share/confd/
+     |- factory.d/
+     |  |- 10-foo.json
+     |  |- 10-bar.json
+     |  `- 10-qux.json
+     `- failure.d/
+        |- 10-xyzzy.json
+        `- 10-garply.json
+
+To override, or extend, these files in you br2-external, set up a rootfs
+overlay and add it last in `BR2_ROOTFS_OVERLAY`.  Your overlay can look
+something like this:
+
+    ./board/common/rootfs/
+      |- etc/
+      |  |- confdrc             # See below
+      |  `- confdrc.local
+      `- usr/
+         `- share/
+            `- confd/
+               |- 10-foo.json   # Override Infix foo
+               |- 30-bar.json   # Extend, probably 10-bar.json
+               `- 30-fred.json  # Extend, your own defaults
+
+Using the same filename in your overlay, here `10-foo.json`, completely
+replaces the contents of the same file provided by Infix.  If you just
+want to extend, or replace parts of an Infix default, use `30-....json`.
+Here the file `30-bar.json` is just a helpful hit to maintainers of your
+br2-external that it probably extends Infix' `10-bar.json`.
+
+The reason for the jump in numbers is that 20 is reserved for files
+generated by Infix' `gen-function` scripts.  Your br2-external can
+provide a few custom ones that the `bootstrap` knows about, e.g.,
+`gen-ifs-custom` that overrides `20-interfaces.json`.  See the
+bootstrap script for more help, and up-to-date information.
+
+> **Note:** you may not need to provide your own `/etc/confdrc`.  The
+> one installed by `confd` is usually enough.  However, if you want to
+> adjust the behavior of `bootstrap` you may want to override it.  There
+> is also `confdrc.local`, which usually is enough to change arguments
+> to scripts like `gen-interfaces`, e.g., to create a bridge by default,
+> you may want to look into `GEN_IFACE_OPTS`.
+
+
 Integration
 -----------