Use cleanup_flavor in setup-baremetal.

Adds an new cleanup_flavor function which we can use
to cleanup flavors a bit more nicely. This avoids
having setup-baremetal print out this nasty error
the first time it runs:

Change-Id: Ic237d4436b609a1ae08747a5b5d384b5da4909be
ERROR: No flavor with a name or ID of 'baremetal' exists.