stlink: support of ST-LINK TCP server using stlink-dap and hla

Quote: The ST-LINK TCP server is an application to share the debug
interface of a single ST-LINK board among several host applications,
typically a debugging tool and a monitoring tool.

Note: ST-Link TCP server does not support the SWIM transport.

ST-LINK TCP server allows several applications to connect to the same
ST-Link through sockets (TCP).

To use ST-LINK TCP server:
 - using stlink-dap : use 'st-link backend tcp [port]'
 - using hla : use 'hla_stlink_backend tcp [port]'

the default port value is 7184

Change-Id: I9b79f65267f04b1e978709934892160e65bd2d6d
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5633
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>