This prepares using mb as a library and not as a sub
process. It is used like that in the new list command that
shows available configurations.

Also added more robust checks of user inputs, better hints
and better docu.

BUG=chromium:625791
NOTRY=true

Review-Url: https://codereview.chromium.org/2300333002
Cr-Commit-Position: refs/heads/master@{#39136}