cmd gpt: test in sandbox

Make minor changes to README.gpt and sandbox_defconfig to support
testing of the gpt command's functionality in the sandbox.
Signed-off-by: NAlison Chaiken <alison@peloton-tech.com>

doc/README.gpt

@@ -210,6 +210,16 @@ U-BOOT> gpt verify mmc 0 $partitions
 U-BOOT> if test $? = 0; then echo "GPT OK"; else echo "GPT ERR"; fi
 
 
+The GPT functionality may be tested with the 'sandbox' board by
+creating a disk image as described under 'Block Device Emulation' in
+board/sandbox/README.sandbox:
+
+=>host bind 0 ./disk.raw
+=> gpt read host 0
+[ . . . ]
+=> gpt flip host 0
+[ . . . ]
+
 Partition type GUID:
 ====================