binman: capsule: Add support for generating EFI capsules

Add support in binman for generating EFI capsules. The capsule
parameters can be specified through the capsule binman entry. Also add
test cases in binman for testing capsule generation.

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