efi: Create a new CONFIG_EFI

Create a Kconfig which indicates that EFI functionality is in use,
either as a client (EFI app / stub) or provider (EFI loader). This will
make it easier to share code between these two parts of U-Boot

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/efi/Kconfig

@@ -0,0 +1,12 @@
+# SPDX-License-Identifier: GPL-2.0-or-later
+#
+# Copyright 2025 Simon Glass <sjg@chromium.org>
+#
+
+config EFI
+	bool
+	help
+	  Indicates that EFI functionality is enabled, either via EFI_CLIENT or
+	  EFI_LOADER
+
+	  This is used to provide libraries shared by both.