opkg must have one sub-command argument
usage: opkg [options...] sub-command [arguments...]
where sub-command is one of:
update Update list of available packages
upgrade <pkgs> Upgrade packages
install <pkgs> Install package(s)
configure <pkgs> Configure unpacked package(s)
remove <pkgs|regexp> Remove package(s)
flag <flag> <pkgs> Flag package(s)
<flag>=hold|noprune|user|ok|installed|unpacked (one per invocation)