adb: make 'wait-for-{recovery,sideload,bootloader} work.

The help for wait-for-* documents this as working, but we only had a
special case for 'wait-for-device'. Make the others work as well.

Bug: http://b/28170823
Change-Id: I57d70f8b9a383947c4f761919bfbd8515a013520