binman: Fix subentry expansion for FIT entry type

Binman tries to expand some entries into parts that make it up, e.g.
'u-boot' into a 'u-boot-expanded' section that contains 'u-boot-nodtb'
and 'u-boot-dtb'. Entries with child entries must call ExpandEntries()
on them to build a correct image, as it's possible that unexpanded child
entries have no data of their own. The FIT entry type doesn't currently
do this, which means putting a "u-boot" entry inside it doesn't work as
expected.

Implement ExpandEntries() for FIT and add a copy of a simple FIT image
test that checks subentry expansion in FIT entries.

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