kconfig: change file_lookup() to return the file name

Currently, file_lookup() returns a pointer to (struct file), but the
callers use only file->name.

Make it return the ->name member directly.

This adjustment encapsulates struct file and file_list as internal
implementation.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/kconfig/menu.c

@@ -16,8 +16,6 @@ static const char nohelp_text[] = "There is no help available for this option.";
 struct menu rootmenu;
 static struct menu **last_entry_ptr;
 
-struct file *file_list;
-
 void menu_warn(struct menu *menu, const char *fmt, ...)
 {
 	va_list ap;