efi_loader: Enable uefi capsule authentication

Add support for enabling uefi capsule authentication. This feature is
enabled by setting the environment variable
"capsule_authentication_enabled".

The following configs are needed for enabling uefi capsule update and
capsule authentication features on the platform.

CONFIG_EFI_HAVE_CAPSULE_SUPPORT=y
CONFIG_EFI_CAPSULE_ON_DISK=y
CONFIG_EFI_CAPSULE_FIRMWARE_MANAGEMENT=y
CONFIG_EFI_CAPSULE_FIRMWARE=y
CONFIG_EFI_CAPSULE_FIRMWARE_RAW=y
CONFIG_EFI_CAPSULE_AUTHENTICATE=y

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>