The reference manual makes a reference to EXTRA_IMAGE_FEATURES and it appears in the local.conf file, but it is not documented that you can use anything in IMAGE_FEATURES. It should get described in the variable list as well as a mention in Appendix E
I edited the Poky Reference Manual in several areas to include information about EXTRA_IMAGE_FEATURES. This variable was virtually ignored in the current version of the manual. It now has a glossary entry and gets some attention throughout the manual where it talks about customising image features. Primarily the focus was on the IMAGE_FEATURES variable. This bug can be marked resolved when these changes are merged into poky/master.
http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=72ed33f17966efba6b40aa751052ad0a04010006
Fixes in manual.