There no _single_ command or argument, but you can easily do it. To upgrade all of the packages, LEDE recommends,
opkg list-upgradable | cut -f 1 -d ' ' | xargs -r opkg upgrade
There are other less efficient ways where people use AWK and such.
An **important caveat** often follows with extensive use of LEDE / OpenWRT's `opkg`
> Since OpenWrt firmware stores the base system in a compressed read-only partition, **any update to base system packages will be written in the read-write partition and therefore use more space than it would if it was just overwriting the older version in the compressed base system partition.** It's recommended to check the available space in internal flash memory and the space requirements for updates of base system packages.