Add initial RTT support

Real Time Transfer (RTT) is an interface specified by SEGGER based on
basic memory reads and writes to transfer data bidirectionally between
target and host.
Every target that supports so called "background memory access", which
means that the target memory can be accessed by the debugger while the
target is running, can be used.

RTT is especially of interest for targets which do not support Serial
Wire Output (SWO) (e.g. ARM Cortex-M0) or where using semihosting is
not possible (e.g. real-time applications) [1].

The data transfer is organized in channels where each channel consists
of an up- and/or down-channel. See [2] for more details.

Channels are exposed via TCP connections. One or more RTT server can be
assigned to each channel to make them accessible to an unlimited number
of TCP connections.

The current implementation does not respect buffer flags which are used
to determine what happens when writing to a full buffer.

Note that the implementation is designed in a way that the RTT
operations can be directly performed by an adapter (e.g. J-Link).

[1] https://devzone.nordicsemi.com/tutorials/6/
[2] https://www.segger.com/jlink-rtt.html

Change-Id: I8bc8a1b381fb74e08b8752d5cf53804cc573c1e0
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/4055
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>

doc/openocd.texi

@@ -8423,6 +8423,94 @@ the watchpoint should trigger. The value may be first be masked
 using @var{mask} to mark ``don't care'' fields.
 @end deffn
 
+
+@section Real Time Transfer (RTT)
+
+Real Time Transfer (RTT) is an interface specified by SEGGER based on basic
+memory reads and writes to transfer data bidirectionally between target and host.
+The specification is independent of the target architecture.
+Every target that supports so called "background memory access", which means
+that the target memory can be accessed by the debugger while the target is
+running, can be used.
+This interface is especially of interest for targets without
+Serial Wire Output (SWO), such as ARM Cortex-M0, or where semihosting is not
+applicable because of real-time constraints.
+
+@quotation Note
+The current implementation supports only single target devices.
+@end quotation
+
+The data transfer between host and target device is organized through
+unidirectional up/down-channels for target-to-host and host-to-target
+communication, respectively.
+
+@quotation Note
+The current implementation does not respect channel buffer flags.
+They are used to determine what happens when writing to a full buffer, for
+example.
+@end quotation
+
+Channels are exposed via raw TCP/IP connections. One or more RTT servers can be
+assigned to each channel to make them accessible to an unlimited number
+of TCP/IP connections.
+
+@deffn Command {rtt setup} address size ID
+Configure RTT for the currently selected target.
+Once RTT is started, OpenOCD searches for a control block with the
+identifier @var{ID} starting at the memory address @var{address} within the next
+@var{size} bytes.
+@end deffn
+
+@deffn Command {rtt start}
+Start RTT.
+If the control block location is not known, OpenOCD starts searching for it.
+@end deffn
+
+@deffn Command {rtt stop}
+Stop RTT.
+@end deffn
+
+@deffn Command {rtt polling_interval [interval]}
+Display the polling interval.
+If @var{interval} is provided, set the polling interval.
+The polling interval determines (in milliseconds) how often the up-channels are
+checked for new data.
+@end deffn
+
+@deffn Command {rtt channels}
+Display a list of all channels and their properties.
+@end deffn
+
+@deffn Command {rtt channellist}
+Return a list of all channels and their properties as Tcl list.
+The list can be manipulated easily from within scripts.
+@end deffn
+
+@deffn Command {rtt server start} port channel
+Start a TCP server on @var{port} for the channel @var{channel}.
+@end deffn
+
+@deffn Command {rtt server stop} port
+Stop the TCP sever with port @var{port}.
+@end deffn
+
+The following example shows how to setup RTT using the SEGGER RTT implementation
+on the target device.
+
+@example
+resume
+
+rtt setup 0x20000000 2048 "SEGGER RTT"
+rtt start
+
+rtt server start 9090 0
+@end example
+
+In this example, OpenOCD searches the control block with the ID "SEGGER RTT"
+starting at 0x20000000 for 2048 bytes. The RTT channel 0 is exposed through the
+TCP/IP port 9090.
+
+
 @section Misc Commands
 
 @cindex profiling