efi_loader, pytest: add UEFI secure boot tests (image)

Provide test cases for
 * image authentication for signed images
   (test_efi_secboot/test_signed.py)
 * image authentication for unsigned images
   (test_efi_secboot/test_unsigned.py)

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>