tremor: fix svn checkout

Commit 1dc19c44 (split tremor into its own package) unfortunately
broke the svn checkout step (but normally not noticable as we have
a tarball on sources.buildroot.net that will get downloaded instead).

Fix it by using a custom download step, and remove unused variables
while we're at it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>