GPT: create block device for sandbox testing

Provide a Python function that creates a small block device for the
purpose of testing the cmd/gpt.c or cmd/part.c functions in the u-boot
sandbox.

Signed-off-by: Alison Chaiken <alison@peloton-tech.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/board/sandbox/README.sandbox b/board/sandbox/README.sandbox
index 2e2c819..9bc13e1 100644
--- a/board/sandbox/README.sandbox
+++ b/board/sandbox/README.sandbox
@@ -337,6 +337,11 @@
 $> sudo mkfs.vfat -n EFI -v ${lodev}p1
 $> sudo mkfs.ext4 -L ROOT -v ${lodev}p2
 
+or utilize the device described in test/py/make_test_disk.py:
+
+   #!/usr/bin/python
+   import make_test_disk
+   make_test_disk.makeDisk()
 
 Writing Sandbox Drivers
 -----------------------