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

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