feat(imgutils): give riscv64 Ubuntu netboot images their network drivers

The Ubuntu driver table had no riscv64 entry, so genimage was handed an
empty list and built an image carrying no network module. A node whose NIC
is not built into the kernel then has no interface to fetch its root
filesystem with. The architecture now gets the same drivers the enterprise
Linux table lists for it, plus the overlay module every Ubuntu image needs.

Signed-off-by: Vinícius Ferrão <2031761+viniciusferrao@users.noreply.github.com>

xCAT-server/share/xcat/netboot/imgutils/imgutils.pm

@@ -278,6 +278,7 @@ sub default_net_drivers {
             x86_64 => [qw(tg3 bnx2 bnx2x e1000 e1000e igb mlx_en mlx5_core virtio_net overlay)],
             ppc64el => [qw(tg3 bnx2 bnx2x e1000 e1000e igb ibmveth ehea mlx_en mlx4_en mlx5_core virtio_net overlay)],
             ppc64   => [qw(e1000 e1000e igb ibmveth ehea)],
+            riscv64 => [qw(e1000 e1000e igb ixgbe r8169 tg3 bnx2x mlx5_core virtio_net overlay)],
             s390x   => [qw(qdio ccwgroup)],
         },
     );