jlink: Add EMUCOM support

EMUCOM is a way to communicate with a J-Link device via so called
channels. A channel can either be read or written in a single
operation.

Beside the reserved channels for SEGGER, there are channels available to
implement vendor and/or device specific functionalities. For example,
EMUCOM is used on many starter and development kits from Silicon Labs to
access power measurements and various other information and settings.

Change-Id: I6094109c043b34aed4a40ceabe71f30ff896bf1d
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3794
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

doc/openocd.texi

@@ -2702,6 +2702,26 @@ Reset the current configuration.
 @deffn {Command} {jlink config write}
 Write the current configuration to the internal persistent storage.
 @end deffn
+@deffn {Command} {jlink emucom write <channel> <data>}
+Write data to an EMUCOM channel. The data needs to be encoded as hexadecimal
+pairs.
+
+The following example shows how to write the three bytes 0xaa, 0x0b and 0x23 to
+the EMUCOM channel 0x10:
+@example
+> jlink emucom write 0x10 aa0b23
+@end example
+@end deffn
+@deffn {Command} {jlink emucom read <channel> <length>}
+Read data from an EMUCOM channel. The read data is encoded as hexadecimal
+pairs.
+
+The following example shows how to read 4 bytes from the EMUCOM channel 0x0:
+@example
+> jlink emucom read 0x0 4
+77a90000
+@end example
+@end deffn
 @deffn {Config} {jlink usb} <@option{0} to @option{3}>
 Set the USB address of the interface, in case more than one adapter is connected
 to the host. If not specified, USB addresses are not considered. Device