pkg-generic: make PKG_DL_DIR equal to DL_DIR

pkg-generic: make PKG_DL_DIR equal to DL_DIR

This variable is currently not used by anyone.
The value is changed to match the path to DL_DIR.

The next patch will introduce the use of PKG_DL_DIR for packages that
use specific EXTRACT_CMDS.

And then we will be able to change the value of PKG_DL_DIR to create a
new directory structure in DL_DIR with a subdirectory for each package.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>