binman: Fix up a few missing comments

Tidy up a few test functions which lack argument comments. Rename one that
has the same name as a different test.

Also fix up the comment for PrepareImagesAndDtbs().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/control.py b/tools/binman/control.py
index 3b52326..15bfac5 100644
--- a/tools/binman/control.py
+++ b/tools/binman/control.py
@@ -346,6 +346,11 @@
         dtb_fname: Filename of the device tree file to use (.dts or .dtb)
         selected_images: List of images to output, or None for all
         update_fdt: True to update the FDT wth entry offsets, etc.
+
+    Returns:
+        OrderedDict of images:
+            key: Image name (str)
+            value: Image object
     """
     # Import these here in case libfdt.py is not available, in which case
     # the above help option still works.