binman: Add a new "skip-at-start" property in Section class

Currently binman calculates '_skip_at_start' based on 'end-at-4gb'
property and it is used for x86 images.

For PowerPC mpc85xx based CPU, CONFIG_SYS_TEXT_BASE is the entry
offset of the first entry. It can be 0xeff40000 or 0xfff40000 for
nor flash boot, 0x201000 for sd boot etc, so "_skip_at_start"
should be set to CONFIG_SYS_TEXT_BASE.

'end-at-4gb' property is not applicable where CONFIG_SYS_TEXT_BASE +
Image size != 4gb.

Add new property 'skip-at-start' in Section class so that
'_skip_at_start' can be calculated either based on 'end-at-4gb'
or based on "skip-at-start".

Add a test case to check that 'skip-at-start' and 'end-at-4gb'
property can't be used together.

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>
diff --git a/tools/binman/ftest.py b/tools/binman/ftest.py
index a8456c2..36519a2 100644
--- a/tools/binman/ftest.py
+++ b/tools/binman/ftest.py
@@ -711,6 +711,14 @@
         self.assertIn("Section '/binman': Section size must be provided when "
                       "using end-at-4gb", str(e.exception))
 
+    def test4gbAndSkipAtStartTogether(self):
+        """Test that the end-at-4gb and skip-at-size property can't be used
+        together"""
+        with self.assertRaises(ValueError) as e:
+            self._DoTestFile('80_4gb_and_skip_at_start_together.dts')
+        self.assertIn("Section '/binman': Provide either 'end-at-4gb' or "
+                      "'skip-at-start'", str(e.exception))
+
     def testPackX86RomOutside(self):
         """Test that the end-at-4gb property checks for offset boundaries"""
         with self.assertRaises(ValueError) as e: