Add adapter plugin for simplejtag level0 protocol.

https://github.com/pfalcon/simplejtag
Add missing turn clock at the end of read word.
SWDAdapterBase: Fix typo in var name.
For convenience, strip ".py" from passed in adapter name.
DebugPort: rewrite to use new API and symbolic constants.
DebugPort: Rework init logic.
swd-identify: use DebugPort API.