Squashed 'external/devlib/' changes from 0a910071f..45aebdaca

45aebdaca connection: Ensure we don't leak too many BackgroundCommand
1239fd922 connection: Make BackgroundCommand deregister itself
069d2322f connection: Add BackgroundCommand.__init__(conn)
7bdd6a0ad connection: Terminate background commands on close()
27fb0453a target: Fix and generalize Target.kick_off()
9e0300b9f shutils: Fix broken redirections
e0c53d099 ftrace: Set top-level buffer size

git-subtree-dir: external/devlib
git-subtree-split: 45aebdaca9ca7efeaa544c268e1a54cdc424f661