The poky.conf file tracks all the distros supported by the Yocto Project. (http://git.yoctoproject.org/cgit/cgit.cgi/poky/tree/meta-poky/conf/distro/poky.conf) The supported, and subsequently sanity tested, distros are listed in the "SANITY_TEST_DISTROS" section in poky.conf. Each supported distro is deployed and used as a Yocto Autobuilder build worker. For more information on supported distros, please refer to the "Supported Linux Distributions" section in the Yocto Project Reference Manual.
Example of distro list in poky.conf: SANITY_TESTED_DISTROS ?= " \ poky-2.4 \n \ poky-2.5 \n \ ubuntu-15.04 \n \ ubuntu-16.04 \n \ ubuntu-16.10 \n \ ubuntu-17.04 \n \ fedora-26 \n \ centos-7 \n \ debian-8 \n \ debian-9 \n \ opensuse-42.1 \n \ opensuse-42.2 \n \ "
Again, the previous listing is just an example and is not meant to be a definitive reference.
How to add a new entry or additional distro information
The criteria for adding a supported distro is quite rigid. Obviously, any candidate distro must be of production quality. In other words, the distro is capable of being used in a production environment on a Yocto Project Autobuilder build worker. A compelling reason must also exist for the Yocto Project to actually use the distro on a build worker. Obscure, poorly supported, or highly customized distros are not likely to be good candidates for submission. Major stable distros (e.g. Debian, Fedora, Ubunto, Centos, and so forth.) are generally already supported. If they are not supported, they are likely on the list to become supported once sufficient criteria are met. Please be sure to check the latest supported distros beforehand to make sure the distro in question is not already supported.
If you have a compelling reason to suggest a distro for support, please submit your request to the Yocto Project mailing list for discussion. You will likely be asked to provide the reasoning behind your request. Along with your request, relevant bug reports, failed test cases, and so forth are helpful. Have them ready in advance.
If you are not already a member of the Yocto Project mailing list, you can find more information and subscription requirements here: https://lists.yoctoproject.org/listinfo/yocto.
The Yocto Project mailing list is also the right place for more general questions or issues surrounding distro support.
Distro test coverage
Distro Testing is intended to reveal bugs that are distribution-specific using the yocto-autobuilder. The tests all run on identical hardware and use each updated OS.
For detailed information on Distro testing, please see Distro_Testing_Plan.
For more information on the overall QA testing plan, please see QA_Master_Test_Plan. (Comprehensive)