efi: Move default filename to a function

Use a function to obtain the device EFI filename, so that we can control
how sandbox behaves.

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

include/efi.h

@@ -669,4 +669,13 @@ int efi_get_mmap(struct efi_mem_desc **descp, int *sizep, uint *keyp,
  */
 void efi_show_tables(struct efi_system_table *systab);
 
+/**
+ * efi_get_basename() - Get the default filename to use when loading
+ *
+ * E.g. this function returns BOOTAA64.EFI for an aarch target
+ *
+ * Return: Default EFI filename
+ */
+const char *efi_get_basename(void);
+
 #endif /* _LINUX_EFI_H */