Revert "pkg-download: remove explicit PKG_VERSION from github helper"

Revert "pkg-download: remove explicit PKG_VERSION from github helper"

This reverts commit 1e5a8916b2ab4c9c99548fa6fbd4855eee323881.

The idea was that the version string can be derived because we know the
package name.

However, this patch does not account for the fact that $(pkgname) always
points to the latest pacakge scanned, which in all other situation we're
using it, is the current package, because it is used inside one ot he
xxx-inner macros that are $(eval)ualed. So $(pkgname) is evaluated
"early" and gets the expected value.

However, the github value is not in one of those macros, so it gets
evaluated "late", when doing the actual download. So, by that time,
$(pkgname) will expand to the last package scanned, which is actuall the
manual (without a br2-external tree).

That would require that the _SITE variable be assigned with the :=
assignment operator. This is weird, because that would make it the only
variable to require that, but only when using the github helper, which
is even less obvious and would cause a lot of trouble...

The obvious fixup would seem to be to use $(PKG) instead, because that
already contains the upper-case package name that vcan be used as a
prefix to variables.

However, that does not work either, because we have a check that forbids
a trsailing slash in _SITE, check that is done in pacakge/pkg-generic,
inside the xxx-inner macro, during the $(eval) call.

And at that time, PKG is not yet defined, because it is only defined for
an actual recipe.

So we can't seem to have a workable solution. So, just revert the patch.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>