binman: Refuse to replace sections for now

Binman interfaces allow attempts to replace any entry in the image with
arbitrary data. When trying to replace sections, the changes in the
section entry's data are not propagated to its child entries. This,
combined with how sections rebuild their contents from its children,
eventually causes the replaced contents to be silently overwritten by
rebuilt contents equivalent to the original data.

Add a simple test for replacing a section that is currently failing due
to this behaviour, and mark it as an expected failure. Also, raise an
error when replacing a section instead of silently pretending it was
replaced.

Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>