sandbox: fix bloblist_finish() linker error if BLOBLIST=n

Having CONFIG_OF_EMBED=y && CONFIG_BLOBLIST=n leads to the link
error:
```
ld: /tmp/ccwtRVty.ltrans0.ltrans.o: in function `state_uninit':
arch/sandbox/cpu/state.c:508: undefined reference to `bloblist_finish'
```

Fixes: 1c52fcca72 ("sandbox: Write out bloblist when exiting")
Signed-off-by: Evgeny Bachinin <EABachinin@salutedevices.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
(cherry picked from commit e5178be686)

include/bloblist.h

@@ -388,6 +388,7 @@ int bloblist_new(ulong addr, uint size, uint flags, uint align_log2);
  */
 int bloblist_check(ulong addr, uint size);
 
+#if CONFIG_IS_ENABLED(BLOBLIST)
 /**
  * bloblist_finish() - Set up the bloblist for the next U-Boot part
  *
@@ -397,6 +398,12 @@ int bloblist_check(ulong addr, uint size);
  * Return: 0
  */
 int bloblist_finish(void);
+#else
+static inline int bloblist_finish(void)
+{
+	return 0;
+}
+#endif /* BLOBLIST */
 
 /**
  * bloblist_get_stats() - Get information about the bloblist