Zephyr images can run on various hardware platforms. Some of these do not have native yocto/oe-core support. In order to support platforms with Altera/Nios2 we need the corresponding toolchain.
There is support for Altera/Nios2 toolchain in Zephyr SDK, so it should be straightforward to re-use the recipes.
Merged with master in meta-zephyr: https://git.yoctoproject.org/cgit/cgit.cgi/meta-zephyr/commit/?id=d7dcbd54bdab9f73cf52da73584fac81305ba020