wget: Avoid bad checksum race issues
    
    If two recipes have conflicting checksums for a file, the code will currently
    remove the existing file when a mismatch is downloaded, even if another task
    successfully fetched it.
    
    This changes the code to verify the checksum (if possible) before replacing
    the file. This removes a potential race window and stops builds failing
    everywhere from one incorrect checksum.
    
    To make this work, we need to be able to override localpath and avoid
    NoChecksum errors being logged.
    
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>