Makefile: add BR2_DL_DIR to EXTRA_ENV

Also export BR2_DL_DIR for incoming download helper scripts.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/Makefile.in

@@ -305,6 +305,7 @@ HOST_MAKE_ENV=PATH=$(BR_PATH) \
 # post-images)
 EXTRA_ENV=\
 	PATH=$(BR_PATH) \
+	BR2_DL_DIR=$(BR2_DL_DIR) \
 	BUILD_DIR=$(BUILD_DIR)
 
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