binman: Automatically expand phase binaries into sections

When creating an entry, check for an expanded version of that entry, then
use it instead. This allows, for example use of:

   u-boot {
   };

instead of having to write out in full:

   u-boot {
      type = "section";

      u-boot-nodtb {
      };

      u-boot-dtb {
      };
   };

Add an implementaion of this and associated documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>