package/yaffs2utils: fix upstream URL

package/yaffs2utils: fix upstream URL

The current URL from which we download the yaffs2utils tarball no
longer works:

-2019-11-02 10:17:20--  https://yaffs2utils.googlecode.com/files/0.2.9.tar.gz
Resolving yaffs2utils.googlecode.com (yaffs2utils.googlecode.com)... 2a00:1450:400c:c02::52, 173.194.76.82
Connecting to yaffs2utils.googlecode.com (yaffs2utils.googlecode.com)|2a00:1450:400c:c02::52|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2019-11-02 10:17:20 ERROR 404: Not Found.

So, let's replace it with a working URL.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>