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/lkc.h

@@ -52,7 +52,7 @@ static inline void xfwrite(const void *str, size_t len, size_t count, FILE *out)
 }
 
 /* util.c */
-struct file *file_lookup(const char *name);
+const char *file_lookup(const char *name);
 void *xmalloc(size_t size);
 void *xcalloc(size_t nmemb, size_t size);
 void *xrealloc(void *p, size_t size);