Default device group

[yyoon]

(idea from discussion with @fredquintana)

Consider having a notion of default device group in madb. When the default device group is set, then all the future madb command will only run on that device group, unless the target devices are explicitly provided.

It would be useful if the default device group is not shared across different shells, so that one can have separate shells open for each target devices. This could potentially be done by storing the default device group somewhere in the shell environment variable.