binman: Allow sections to have an offset

At present sections are always placed automatically. Even if an 'offset'
property is provided it is ignored. Update the logic to support an offset
for sections.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/tools/binman/README b/tools/binman/README
index 04ed2b79..927fa85 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -342,6 +342,13 @@
 	Sets the image size in bytes, for example 'size = <0x100000>' for a
 	1MB image.
 
+offset:
+	This is similar to 'offset' in entries, setting the offset of a section
+	within the image or section containing it. The first byte of the section
+	is normally at offset 0. If 'offset' is not provided, binman sets it to
+	the end of the previous region, or the start of the image's entry area
+	(normally 0) if there is no previous region.
+
 align-size:
 	This sets the alignment of the image size. For example, to ensure
 	that the image ends on a 512-byte boundary, use 'align-size = <512>'.