There's a useful thread over at StackExchange that lists options one has to make this happen. Note that blindly doing this vs selectively updating packages will have some unintended consequence of using up more space than is required.
My own workaround here is to remove all the unnecessary stock packages you don't use in your device in the first place (like all the language translations).
This line has, so far, worked for me while upgrading FriendlyWRT in the NanoPi R5S:
opkg list-upgradable | cut -f 1 -d ' ' | xargs -r opkg upgrade
This alternative looks to be a viable candidate if you get errors:
opkg list-upgradable | cut -f 1 -d ' ' | xargs opkg install