sandbox: Add a dummy booti command

Add basic sandbox support for 'booti' so we can start to boot the test
ARMbian image. This is helpful in checking that it is parsed correctly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/tools-only_defconfig b/configs/tools-only_defconfig
index fa1ff4a..b54d2ce 100644
--- a/configs/tools-only_defconfig
+++ b/configs/tools-only_defconfig
@@ -16,6 +16,7 @@
 CONFIG_BOOTCOMMAND="run distro_bootcmd"
 # CONFIG_CMD_BOOTD is not set
 # CONFIG_CMD_BOOTM is not set
+# CONFIG_CMD_BOOTI is not set
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_EXTENSION is not set
 # CONFIG_CMD_DATE is not set