pkg-download: remove explicit PKG_VERSION from github helper

pkg-download: remove explicit PKG_VERSION from github helper

Currently it is always required to add package version as an argument to
the github helper. Since the version is always defined as PKG_VERSION,
drop this argument and generate it automatically inside the helper
routine.

The github helper function is extended to support both 2 and 3 argument
variants (ie. either use the provided package version argument or
automatically substitute with PKG_VERSION if not available), which can
make the transition of the package files easier as well allows using the
3-argument variant outside of package definitions.

Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>