binman: Allow entries to expand after packing

Add support for detecting entries that change size after they have already
been packed, and re-running packing when it happens.

This removes the limitation that entry size cannot change after
PackEntries() is called.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/binman/README b/tools/binman/README
index abbf809..9860633 100644
--- a/tools/binman/README
+++ b/tools/binman/README
@@ -566,7 +566,8 @@
 The default implementatoin does nothing. This can be overriden to adjust the
 contents of an entry in some way. For example, it would be possible to create
 an entry containing a hash of the contents of some other entries. At this
-stage the offset and size of entries should not be adjusted.
+stage the offset and size of entries should not be adjusted unless absolutely
+necessary, since it requires a repack (going back to PackEntries()).
 
 10. WriteSymbols() - write the value of symbols into the U-Boot SPL binary.
 See 'Access to binman entry offsets at run time' below for a description of