manual: add documentation about DL_DIR

And at the same time don't encourage use of BR2_DL_DIR

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

docs/manual/adding-packages-generic.txt

@@ -263,7 +263,7 @@ information is (assuming the package name is +libfoo+) :
   the file using this URL. Otherwise, Buildroot will assume the file
   to be downloaded is located at +LIBFOO_SITE+. Buildroot will not do
   anything with those additional files, except download them: it will
-  be up to the package recipe to use them from +$(BR2_DL_DIR)+.
+  be up to the package recipe to use them from +$(DL_DIR)+.
 
 * +LIBFOO_SITE_METHOD+ determines the method used to fetch or copy the
   package source code. In many cases, Buildroot guesses the method
@@ -537,6 +537,9 @@ In the action definitions, you can use the following variables:
 * +$(@D)+, which contains the directory in which the package source
   code has been uncompressed.
 
+* +$(DL_DIR)+ contains the path to the directory where all the downloads made
+  by Buildroot are stored.
+
 * +$(TARGET_CC)+, +$(TARGET_LD)+, etc. to get the target
   cross-compilation utilities