radvd: switch to new patch naming convention, use Git formatted patches

This commit renames the radvd patches to follow the new naming
convention, and reformats them to be Git patches.

Since the patches are renamed, we adjust the comment in the .mk file
explaining why we autoreconf the package.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>