core: do not attempt downloads with no _VERSION set

core: do not attempt downloads with no _VERSION set

Currently, when the package defines no _SOURCE and no _VERSION
variables, we end up trying to download <PACKAGE>-undefined.tar.gz.

This is utterly pointless.

Change the logic to not attempt a download when neither _SOURCE
nor _VERSION is set:
  - do not set an undefined _VERSION to "undefined"
  - do not append an empty (or "undefined") version to the package
    rawname

Consequently, for packages that have no _VERSION, the build
directory will be just the package name (which to some may look
nicer than the current "package-undefined").

Furthermore, the message trace is also a bit leaner (to the same
people that find "package" nicer than "package-undefined").

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>