efi: Add a command to help with shim

Shim is a program which normally comes before U-Boot in the boot
process. But when the app runs first, it can sometimes chain Shim since
that is what is contained within the bootaa64.efi file, for example.

Add a simple command for dealing with shim. For now it only supports
enabling verbosity.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <sjg@chromium.org>

include/efi.h

@@ -163,6 +163,9 @@ extern const efi_guid_t efi_guid_component_name2;
 /* GUIDs for authentication - most of these are still in efi_loader.h */
 extern const efi_guid_t efi_guid_image_security_database;
 
+/* Access to Shim variables */
+extern const efi_guid_t efi_shim_lock;
+
 /* Generic EFI table header */
 struct efi_table_hdr {
 	u64 signature;