sandbox: Add 'sb' command to access filesystem features

The new 'sb' command is intended to deal with sandbox-specific features
that have no parallel in other archs. This commit adds two sub-commands
to list a directory and read a file from the host filesystem.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>
diff --git a/README b/README
index 51eb71b..42544ce 100644
--- a/README
+++ b/README
@@ -877,6 +877,7 @@
 		CONFIG_CMD_READ		* Read raw data from partition
 		CONFIG_CMD_REGINFO	* Register dump
 		CONFIG_CMD_RUN		  run command in env variable
+		CONFIG_CMD_SANDBOX	* sb command to access sandbox features
 		CONFIG_CMD_SAVES	* save S record dump
 		CONFIG_CMD_SCSI		* SCSI Support
 		CONFIG_CMD_SDRAM	* print SDRAM configuration information