Add STlink terminal draft

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
working st-term proof-of-concept. Period.

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
st-term: handle two signals and fixups

Signed-off-by: Andrew 'Necromant' Andrianov <andrew@ncrmnt.org>
Merge pull request #150 from nekromant/master

serial terminal support