Fsdb annotation method needs to be update

[KyleJeong]

I am happy with Vcd dump, but Fsdb could be better for VCS(with Verdi) debugging.
So I tried to change the Vcd to Fsdb like as.

test().withAnnotations(Seq(WriteFsdbAnnotation)

But it didn't show any errror/warning message. Just nothing.

With the help of Kevin Laeufer at https://gitter.im/freechipsproject/chisel3, I've told to use this one.

test().withAnnotations(Seq(WriteFsdbAnnotation, VcsBackendAnnotation))

There are two suggestions.

• Make error or warning message when user use invalid annotation like as first example
• Add CLI option like as "-DwriteFsb=1"