Fix kick off

[douglas-raillard-arm]

Following @mrkajetanp tests, I updated the PR to use Target.background() instead of relying on nohup and &. The & unfortunately only makes the command a background job in the shell, but adb shell blocks until all background jobs are finished before exiting, making it useless.

Instead, using Target.background() provides the expected behavior. The main change is that background commands are canceled on target disconnect for some connection classes (but not all it seems, we might want to align behaviors there)

[douglas-raillard-arm]

So we should probably:

• deprecate Target.kick_off() as it really is just a Target.background() that leaks which is definitively an anti-pattern
• Align all connection classes to cancel running background commands when the connection is closed. This can be done in generic code.

EDIT: the 2) is implemented there but needs an adjustment before being ready: https://github.com/ARM-software/devlib/pull/626

[mrkajetanp]

I can confirm that with the updated PR the instrument finally works as expected so simply getting rid of kick_off this way is probably a good idea.