MLECO-2881: Minor improvements for downloaded resources

Check could have been bypassed if the script path provided as
an argument was incorrect. Also, a minor update in the CMake
function to fix the path.

Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>
Change-Id: Id3cd4027843783e3ed8700eac503e7ba3f09ad04