efi_selftest: test exit_data

Amend the unit test 'start image exit' to transfer a string as exit data.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/include/efi_selftest.h b/include/efi_selftest.h
index 49d3d6d..dd42e49 100644
--- a/include/efi_selftest.h
+++ b/include/efi_selftest.h
@@ -16,7 +16,7 @@
 
 #define EFI_ST_SUCCESS 0
 #define EFI_ST_FAILURE 1
-
+#define EFI_ST_SUCCESS_STR L"SUCCESS"
 /*
  * Prints a message.
  */