Implement "boot time measurement" test

Boot time is an important indicator for device performance. We should implement a simple "boot time" test measuring this point.

We should also avoid anything likely to affect performance, such as long logs being printed over the serial console, DHCP negociation and so on. This implies we should likely first run a minimal boot test, then restart it by disabling DHCP and adding the `quiet` command-line option.

Relevant topic on the LAVA mailing list: https://lists.lavasoftware.org/archives/list/lava-users@lists.lavasoftware.org/thread/AYYRZA6XJUJC33UZKTBGKGFQ3MGGL3UB/?sort=thread