buildroot

support/testing: large timeout for login prompt

When running multiple instances of emulator in parallel, the login
prompt can take some time to appear.

Use a large timeout when waiting for the prompt to avoid random
failures.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>