test/py: vboot can be run only at Sandbox

Getting this error:
Zynq> sb load hostfs - 100
/home/monstr/data/disk/u-boot/build-zynq_zc706/test.fit
Unknown command 'sb' - try 'help'

because sb command is present only for Sandbox
obj-$(CONFIG_SANDBOX) += host.o

that's why mark this test to be run only at Sandbox

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Simon Glass <sjg@chromium.org>