From the commit message:
    bitbake: cookerdata: Add support for BBFILES_DYNAMIC
    
    BBFILES_DYNAMIC can be used to activate content only when some other
    layers are present. The other layers are identified by the collections
    that they define.
    
    The main use case is to avoid .bbappends without the corresponding .bb
    file in layers that want to modify other layers via .bbappends without
    introducing a hard dependency on those other layers. .bb files could
    also be handled via BBFILES_DYNAMIC.
    
    Entries in BBFILES_DYNAMIC must have the form <collection
    name>:<filename pattern>. Example usage:
     BBFILES_DYNAMIC += " \
         clang-layer:${LAYERDIR}/bbappends/meta-clang/*/*/*.bbappend \
         core:${LAYERDIR}/bbappends/openembedded-core/meta/*/*/*.bbappend \
     "
    
    Parsing is aborted when invalid entries are found with an error
    message like this:
    
     ERROR: BBFILES_DYNAMIC entries must be of the form <collection name>:<filename pattern>, not:
         /work/my-layer/bbappends/meta-security-isafw/*/*/*.bbappend
         /work/my-layer/bbappends/openembedded-core/meta/*/*/*.bbappend
    
    Based on a patch by Richard Purdie.