[jlink] add build option RTT_BUFFER_SIZE to set the RTT buffer size (#…

…938)

When debugging OT via the RTT, OT output logs are often truncated
because the RTT buffer size is too small. This commit adds the
build option RTT_BUFFER_SIZE to easily change the RTT buffer size.

third_party/jlink/CMakeLists.txt

@@ -45,6 +45,13 @@ else()
     )
 endif()
 
+option(RTT_BUFFER_SIZE "sets the RTT up buffer size")
+if(RTT_BUFFER_SIZE)
+    target_compile_options(jlinkrtt PRIVATE
+        -DSEGGER_RTT_CONFIG_BUFFER_SIZE_UP=${RTT_BUFFER_SIZE}
+    )
+endif()
+
 target_compile_options(jlinkrtt PRIVATE
     -DSEGGER_RTT_CONFIG_H=\"${PROJECT_SOURCE_DIR}/third_party/NordicSemiconductor/segger_rtt/SEGGER_RTT_Conf.h\"
     -DUSE_APP_CONFIG=1