binman: Create FIT subentries in the FIT section, not its parent

When reading images from a file, each entry's data is read from its
parent section as specified in the Entry.Create() call that created it.
The FIT entry type has been creating its subentries under its parent
(their grandparent), as creating them under the FIT entry resulted in an
error until FIT was converted into a proper section.

FIT subentries have their offsets relative to the FIT section, and
reading those offsets in the parent section results in wrong data. The
subentries rightfully belong under the FIT entries, so create them
there. Add tests checking that we can extract the correct data for a FIT
entry and its subentries.

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