test: boot: fix bootflow_cmd_label for when DSA_SANDBOX is disabled

test: boot: fix bootflow_cmd_label for when DSA_SANDBOX is disabled

When DSA_SANDBOX is not set, the sandbox tests fail as follows:

 $ ./test/py/test.py --build-dir=$(pwd) -k bootdev_test_any
 [...]
 Scanning for bootflows with label '9'
 [...]
 Cannot find '9' (err=-19)

This is due to the device list containing two less entries than
expected. Therefore, look for label '7' when DSA_SANDBOX is disabled.

The actual use case is NET_LWIP=y (to be introduced in later patches)
which implies DSA_SANDBOX=n for the time being.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>