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/common/Makefile b/common/Makefile
index 54fcc81..719fc23 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -152,6 +152,7 @@
 COBJS-$(CONFIG_CMD_READ) += cmd_read.o
 COBJS-$(CONFIG_CMD_REGINFO) += cmd_reginfo.o
 COBJS-$(CONFIG_CMD_REISER) += cmd_reiser.o
+COBJS-$(CONFIG_SANDBOX) += cmd_sandbox.o
 COBJS-$(CONFIG_CMD_SATA) += cmd_sata.o
 COBJS-$(CONFIG_CMD_SF) += cmd_sf.o
 COBJS-$(CONFIG_CMD_SCSI) += cmd_scsi.o