bzip2: change upstream site

The bzip.org website is down. Use the Buildroot backup download site.
Remove the website link as there is no clear alternative upstream at
this point.

  https://lwn.net/Articles/762264/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>