docs/manual: centralize recommendations about BR2_DL_DIR

We currently have two places where we recommend where BR2_DL_DIR
should be set: "Environment variables" and "Location of downloaded
packages". The former recommends setting BR2_DL_DIR in the .config,
the latter kind of endorses using ~/.bashrc.

We prefer suggesting the ~/.bashrc way since it avoids downloading the
same file multiple times, and anyway it's wise to have all the details
in a unique place. So remove the .config suggestion from "Environment
variables" and let it just point to "Location of downloaded packages".

Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

docs/manual/common-usage.txt

@@ -91,8 +91,9 @@ to +make+ or set in the environment:
   Buildroot stores/retrieves downloaded files
   +
   Note that the Buildroot download directory can also be set from the
-  configuration interface, so through the Buildroot +.config+ file; this
-  is the recommended way of setting it.
+  configuration interface, so through the Buildroot +.config+ file. See
+  xref:download-location[] for more details on how you can set the download
+  directory.
 * +BR2_GRAPH_ALT+, if set and non-empty, to use an alternate color-scheme in
   build-time graphs
 * +BR2_GRAPH_OUT+ to set the filetype of generated graphs, either +pdf+ (the

docs/manual/download-location.txt

@@ -1,6 +1,8 @@
 // -*- mode:doc; -*-
 // vim: set syntax=asciidoc:
 
+[[download-location]]
+
 ==== Location of downloaded packages
 
 The various tarballs that are downloaded by Buildroot are all stored