pkg-generic: introduce _DL_SUBDIR

pkg-generic: introduce _DL_SUBDIR

This per package variable can be used to specify the download
subdirectory used by that package.

The use case here is for example linux-headers and linux, which share
the same sources (because they are the same upstream project), so we
don't want to download twice the kernel, nor store it multiple times
either.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>