Request to add cluster/action to add variables with :append to local.conf

During development, I faced an issue with the variable DISTRO_FEATURES that should always been handled with :append instead of += assignment.

Issue example:

• With DISTRO_FEATURES:append = " pulseaudio" the image is correctly built
• With DISTRO_FEATURES += "pulseaudio" an error rises:

yoctouser@a0df72d896b9:~/workdir/build_corei7$ bb seco-clea-os-image                                                                            │
ERROR: Nothing RPROVIDES 'glmark2' (but /home/yoctouser/workdir/layers/meta-seco/meta-seco-clea-os/recipes-core/packagegroups/packagegroup-seco-benchmark.bb │commit 8d0d02d68eb126d59def9a62c8db14a6e9743de3
RDEPENDS on or otherwise requires it)                                                                                                                        │nicolasparnacci@sa-rd002:/data/nicolasparnacci/CleaOS/package-group-dev/layers/meta-seco/meta-seco-clea-os$ 
glmark2 was skipped: one of 'opengl dispmanx' needs to be in DISTRO_FEATURES                                                                                 │nicolasparnacci@sa-rd002:/data/nicolasparnacci/CleaOS/package-group-dev/layers/meta-seco/meta-seco-clea-os$ 
ERROR: Required build target 'seco-clea-os-image' has no buildable providers.                                                                                │nicolasparnacci@sa-rd002:/data/nicolasparnacci/CleaOS/package-group-dev/layers/meta-seco/meta-seco-clea-os$ git switch -
Missing or unbuildable dependency chain was: ['seco-clea-os-image', 'packagegroup-seco-benchmark', 'glmark2'] 

This happens because += overwrites the default value of DISTRO_FEATURES that is set with ?= in poky/meta-poky/conf/distro/poky.conf.