efi_loader: Kconfig: Select IMAGE_SIGN_INFO when capsule authentication is enabled

Enable building of the crypto helper functions used during capsule
authentication by selecting IMAGE_SIGN_INFO.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>