binman: capsule: Add support for generating EFI empty capsules

Add support in binman for generating EFI empty capsules. These
capsules are used in the FWU A/B update feature. Also add test cases
in binman for the corresponding code coverage.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>