curl: fix the nativesdk ssl enable

The particular interaction of the double override wasn't working for us (we
need these vars adjusted for class-nativesdk+mel, not just one or the other),
so reworked to use a full double override operation, rather than a conditional
append to an override var.

Signed-off-by: Christopher Larson <kergoth@gmail.com>
Merge pull request #132 from kergoth/buildtools-tarball

curl: fix the nativesdk ssl enable