ARC: west runner: mdb: fix unusable console after flash / debug

Currently we try to start MDB (MetaWare Debugger) in background
with ignored sigint no matter if we are planing to do debug
session (via `west debug`) or just run something on HW or on
nSIM (via `west flash`)

That cause really bad user experience as after we do
`west debug` or `west flash` we can't terminate the debugger
from the console. Moreover even if we terminate the debugger
process from separate terminal the console stil would be
broken so we need to do `stty sane` in it to make it usable
again.

Fix all that by proper starting the debugger process.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>