Add dev shell without rootfs

Makes iteration quicker.

fpga/upload.sh

@@ -16,7 +16,11 @@ rsync -a fpga/env.sh "$dst"
 rsync $OPENSBI/share/opensbi/*/fpga/*/firmware/fw_payload.bin "$dst/opensbi.bin"
 rsync "$KERNEL/Image" "$dst/kernel.bin"
 rsync "$INITRD/initrd" "$dst/initrd.bin"
-rsync "$ROOTFS/sd-image/rootfs.img" "$dst/rootfs.img"
+if [ -n "$ROOTFS" ]; then
+  rsync "$ROOTFS/sd-image/rootfs.img" "$dst/rootfs.img"
+else
+  echo "Skipping rootfs"
+fi
 rsync "$UBOOT_ENV" "$dst/uboot.env"
 
 echo "Now go to $dst and run ./boot.sh"