binman: x86: Write skip-at-start when end-at-4gb is used

The end-at-4gb property implies a value for skip-at-start so add it into
the output FDT so that U-Boot can read it.

Now that skip-at-start is implemented, we can drop the workarounds used
in the x86 code to obtain the correct image-pos value.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/x86/lib/fsp2/fsp_init.c b/arch/x86/lib/fsp2/fsp_init.c
index 0be892b..4b5f988 100644
--- a/arch/x86/lib/fsp2/fsp_init.c
+++ b/arch/x86/lib/fsp2/fsp_init.c
@@ -140,7 +140,7 @@
 		if (ret)
 			return log_msg_ret("binman entry", ret);
 		if (!use_spi_flash)
-			rom_offset = map_base + CONFIG_ROM_SIZE;
+			rom_offset = map_base;
 	} else {
 		ret = -ENOENT;
 		if (false)