swim: abstract the transport in stm8 target

swim: abstract the transport in stm8 target

SWIM is implemented by (ab)using the HLA API. This was acceptable
when OpenOCD code did not provided a clear separation between
transports and related APIs. Still today SWIM in OpenOCD is only
supported by STLink, so the decision to re-use the HLA API was the
simpler way to implement it.
After commit efd1d642220a ("adapter: switch from struct
jtag_interface to adapter_driver") the transports API are better
split and SWIM can be implemented as a separate set of API. This
would open the possibility to extend OpenOCD for other adapters
that provide SWIM, e.g. versaloon, or through SPI emulation [1].

Introduce a new set of files swim.[ch] to handle the SWIM API.
Beside the API that almost match the transport low-level data
communication (system_reset, read_mem, write_mem), add a further
API reconnect. Today, inside HLA STLink code, the reconnect is
implemented by hacking the HLA API state(). Please notice that
due to this hack the return type is incorrect; stlink_usb_state()
returns ERROR_OK in SWIM mode, while its return type is enum
target_state. Ignore the type mismatch and still call the HLA API
state in the new SWIM API reconnect. Further commit will fix it.

[1] http://kuku.eu.org/?projects/stm8spi/stm8spi

Change-Id: I52018e1e2200cbd41af8e5031f7b35dc761b61d6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5528
Tested-by: jenkins