binman: Allow updating entries that change size

So far we don't allow entries to change size when repacking. But this is
not very useful since it is common for entries to change size after an
updated binary is built, etc.

Add support for this, respecting the original offset/size/alignment
constraints of the image layout. For this to work the original image
must have been created with the 'allow-repack' property.

This does not support entry types with sub-entries such as files and
CBFS, but it does support sections.

Signed-off-by: Simon Glass <sjg@chromium.org>