fastboot: remove engine.cpp/engine.h

Replace the remnants of engine.cpp with better functionality in
FastBootDriver.  Particularly, add prolog() and epilog()
callbacks to FastBootDriver for printing actions and their result to
the console.  Then clean up the rest of fastboot.cpp to directly use
FastBootDriver.

Test: fastboot works
Change-Id: I0ff01d6a10f75e9dc1c82b46c6e9bb3bc4c68638