package: add a <pkg>_EXTRA_DOWNLOADS variable

Converting the external toolchain logic into a package raises a very
special use case that wasn't handled by the package infrastructure:
the Blackfin toolchain is delivered as two tarballs instead of
one. Unfortunately <pkg>_SOURCE only allows to pass one tarball name.

However, we really want both tarballs to be known by the package
infrastructure, so that the normal 'source' and 'external-deps'
mechanism work fine.

In order to achieve this, we add a <pkg>_EXTRA_DOWNLOADS variable,
which allows a package to list other stuff it would like to see
downloaded, but that are otherwise not used by the package
infrastructure itself: it is up to the package to do it by itself.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>