efi_loader: correct determination of secure boot state

When U-Boot is started we have to use the existing variables to determine
in which secure boot state we are.

* If a platform key PK is present and DeployedMode=1, we are in deployed
  mode.
* If no platform key PK is present and AuditMode=1, we are in audit mode.
* Otherwise if a platform key is present, we are in user mode.
* Otherwise if no platform key is present, we are in setup mode.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>