efi_loader: Add a test app

efi_loader: Add a test app

Add a simple app to use for testing. This is intended to do whatever it
needs to for testing purposes. For now it just prints a message and
exits boot services.

There was a considerable amount of discussion about whether it is OK to
call exit-boot-services and then return to U-Boot. This is not normally
done in a real application, since exit-boot-services is used to
completely disconnect from U-Boot. For now, this part is skipped.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>