infra: introduce github helper function

In order to make it easier to cope with changes in github download-URL's,
this patch introduces the github helper function. It generates the site
URL of a github repository. It's usage is $(call github,user,pkg,version).

[Thomas: extracted from Mischa original commit, macro moved to
pkg-download.mk]

Signed-off-by: Mischa Jonker <mjonker@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>