download: don't overwrite VERSION variable

download: don't overwrite VERSION variable

In package-defaults.mk the VERSION variable contains the final package
version, which is a combination of `PKG_VERSION`, `PKG_RELEASE` and if
used, parameters of `PKG_SOURCE_VERSION`. While this works fine for
building, within the package Makefile the content VERSION varies:

When building a package from a release tarball, the content of VERSION
contains the actual package version, however when building from source
(i.e. Git), the VERSION is overwritten by `PKG_SOURCE_VERSION`.

To fix the overwrite, this commit switches from `VERSION` in download.mk
to `SOURCE_VERSION` which isn't used anywhere else yet.

As a result, Makefiles may pass the package version to be included
inside the binary.

Signed-off-by: Paul Spooren <mail@aparcar.org>