efi_loader, pytest: set up secure boot environment

A fixture for UEFI secure boot tests (image authentication and variable
authentication) is defined. A small file system with test data in a single
partition formatted in fat is created.

This test requires efitools v1.5.2 or later. If the system's efitools
is older, you have to build it on your own and define EFITOOLS_PATH.

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