Suggested improvements to _set_boot_device

Change Iac3a7183db4c3486bd6a2aa1ba36bf33e11722a9 had a number
of non-essential fixups suggested, these have been added in
this dependent patch.

Change-Id: I88c5e46d65f1768765e68d0e87f06172f8e3ebd9
Merge "Suggested improvements to _set_boot_device"