sandbox: Move pre-console buffer out of the way of tracing

These two buffers currently conflict if tracing is enabled. Move the
pre-console buffer and update the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index ed8fac6..48c1e2b 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -477,6 +477,9 @@
       0   CONFIG_SYS_FDT_LOAD_ADDR   Device tree
    e000   CONFIG_BLOBLIST_ADDR       Blob list
   10000   CONFIG_MALLOC_F_ADDR       Early memory allocation
+  f0000   CONFIG_PRE_CON_BUF_ADDR    Pre-console buffer
+ 100000   CONFIG_TRACE_EARLY_ADDR    Early trace buffer (if enabled)
+=
 
 
 --