sandbox: provide static IP addresses for eth{2, 3, 5, 6, 7}

sandbox: provide static IP addresses for eth{2, 3, 5, 6, 7}

The tests in test/dm/eth.c and test/dm/dsa.c use interfaces that have
no static IP addresses configured in the board's default environment
file. That will be a problem when NET_LWIP=y because the lwIP stack
refuses to send ICMP packets through an interface that doesn't have an
IP ("no route to host"). Therefore and in preparation for enabling the
sandbox tests with NET_LWIP, provide such addresses in
board/sandbox/sandbox.env.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>