package/pkg-genric: add possibility to declare download dependencies

package/pkg-genric: add possibility to declare download dependencies

For some packages, we may need to have a certain set of host-tools built
before the download of said packages are attempted. For example, when
the system tar is not suitable, we will want to build our own tar before
we attempt a git download (because we generate a tarball in the git
backend).

Mimick the _EXTRACT_DEPENDENCIES, and introduce _DOWNLOAD_DEPENDENCIES.
As for _EXTRACT_DEPENDENCIES, we do not document _DOWNLOAD_DEPENDENCIES,
on the assumption that it is mostly for internal use.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>