binman: fix passing loadables to mkimage on first run

binman: fix passing loadables to mkimage on first run

FIT use mkimage from BuildSectionData() to build FIT entry contents.
BuildSectionData() get called several times during building FIT image.

Currently when fit inserts loadables, it use self._loadables property that
contain loadables computed during previuos BuildSectionData() invocation.
So for the first run it use empty list and pass no loadables to mkimage.

That makes problem for adding signature to FIT image because mkimage
fails to add signature and aborts building FIT if no loadables provided.

The patch fixes described behaviour in a way that BuildSectionData() uses
recently calculated loadables value, not previosly calculated.

Signed-off-by: Alexander Kochetkov <al.kochet@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>