The semantics of `make debug` and `make debugserver` have changed in
the years since the MSP430 integration. This brings the implementation
back into line with the current semantics
- `make debug` now starts both mspdebug and GDB, no need to
run `make debugserver` prior to `make debug` anymore
- `make debug` no longer flashes the target to not waste flash erase
cycles
- GDB mutliarch support is added
- support for selecting a debug adapter by its serial is added
