binman: Add tests for skip-at-start sections

At present this feature is tested view the end-at-4gb feature. Add some
tests of its own, including the operation of padding.

The third test here shows binman's current, inconsistent approach to
padding in the top-level section. Future patches in this series will
address this.

Signed-off-by: Simon Glass <sjg@chromium.org>
4 files changed