seems meta-acrn have overwritten the default wic image creation plugin and it wont take any values from default OE variables set in yocto which are set for boot image creation. i.e IMAGE_BOOT_FILES variables is ignored or not used. while this variable is widely used in yocto OE core with default wic plugin. reference from here https://github.com/openembedded/openembedded-core/blob/master/scripts/lib/wic/plugins/source/bootimg-partition.py
could you make changes in acrn wic plugin so it follow and inherits the OE plugin variables.