binman: Expand docs and test for padding

Padding becomes part of the entry once the image is written out, but
within binman the entry contents does not include the padding. Add
documentation to make this clear, as well as a test.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index 481b8fd..35c1420 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -3548,12 +3548,39 @@
 
     def testPadInSections(self):
         """Test pad-before, pad-after for entries in sections"""
-        data = self._DoReadFile('166_pad_in_sections.dts')
+        data, _, _, out_dtb_fname = self._DoReadFileDtb(
+            '166_pad_in_sections.dts', update_dtb=True)
         expected = (U_BOOT_DATA + tools.GetBytes(ord('!'), 12) +
                     U_BOOT_DATA + tools.GetBytes(ord('!'), 6) +
                     U_BOOT_DATA)
         self.assertEqual(expected, data)
 
+        dtb = fdt.Fdt(out_dtb_fname)
+        dtb.Scan()
+        props = self._GetPropTree(dtb, ['size', 'image-pos', 'offset'])
+        expected = {
+            'image-pos': 0,
+            'offset': 0,
+            'size': 12 + 6 + 3 * len(U_BOOT_DATA),
+
+            'section:image-pos': 0,
+            'section:offset': 0,
+            'section:size': 12 + 6 + 3 * len(U_BOOT_DATA),
+
+            'section/before:image-pos': 0,
+            'section/before:offset': 0,
+            'section/before:size': len(U_BOOT_DATA),
+
+            'section/u-boot:image-pos': 4,
+            'section/u-boot:offset': 4,
+            'section/u-boot:size': 12 + len(U_BOOT_DATA) + 6,
+
+            'section/after:image-pos': 26,
+            'section/after:offset': 26,
+            'section/after:size': len(U_BOOT_DATA),
+            }
+        self.assertEqual(expected, props)
+
     def testFitImageSubentryAlignment(self):
         """Test relative alignability of FIT image subentries"""
         entry_args = {