binman: Correct fmap output on x86

Normally x86 platforms use the end-at-4gb option. This currently produces
an FMAP with positions which have a large offset. The use of end-at-4gb is
a useful convenience within binman, but we don't really want to export
a map with these offsets.

Fix this by subtracting the 'skip at start' parameter.

Also put the code which convers names to fmap format, for clarity.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/entry.py b/tools/binman/entry.py
index fd72234..01be291 100644
--- a/tools/binman/entry.py
+++ b/tools/binman/entry.py
@@ -197,7 +197,8 @@
         """Set the value of device-tree properties calculated by binman"""
         state.SetInt(self._node, 'offset', self.offset)
         state.SetInt(self._node, 'size', self.size)
-        state.SetInt(self._node, 'image-pos', self.image_pos)
+        state.SetInt(self._node, 'image-pos',
+                       self.image_pos - self.section.GetRootSkipAtStart())
         state.CheckSetHashValue(self._node, self.GetData)
 
     def ProcessFdt(self, fdt):