pkg-generic: implement a <pkg>-external-deps target

This commit implements a new <pkg>-external-deps target in the
pkg-generic package infrastructure. This target simply displays the
list of file names that are needed for <pkg> to build (source tarball,
patches, extra downloads).

In the case of the usage of <pkg>_OVERRIDE_SRCDIR, the path to the
overriden source directory is displayed, prepended by file://. This
matches what the code is currently doing in .stamp_rsync_sourced.

This new target is not currently being used in this commit, but
followup commits will make use of it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>