binman: Convert mkimage to Entry_section

This is needed to handle mkimage with inner section located itself in a
section.

Signed-off-by: Marek Vasut <marex@denx.de>
Use BuildSectionData() instead of ObtainContents(), add tests and a few
other minor fixes:
Signed-off-by: Simon Glass <sjg@chromium.org>
4 files changed