I also ran into this problem, and I believe I had to use the IMAGE_INSTALL_append syntax. There's an example here: i.MX Yocto Project: How do I add an existing package to an image? | Freescale Community.
For the record:
+=operator from the
/conf/local.conffile or from within an image recipe is not recommended as it can cause ordering issues. Since
IMAGE_INSTALLto a default value using the
?=operator, using a
IMAGE_INSTALLwill result in unexpected behavior when used in
conf/local.conf. Furthermore, the same operation from within an image recipe may or may not succeed depending on the specific situation. In both these cases, the behavior is contrary to how most users expect the
+=operator to work.
When you use this variable, it is best to use it as follows:
IMAGE_INSTALL_append = " package-name"