# Pastebin pFg7Jx5O
It seems that you could make things work like this:

mkimage {
   args = "-T", "imx8mimage";

   section {
       // put spl and ddr things in here
       // this will b passed in with -d
    };

   imagename {
      // this data will be passed in with -n
      nxp-imx8mimage-cfg {
          nxp,xx properties here

          // no section {} in here

         // Update BuildSectionData() to return the cfg file as its data
     }