[developer] document serial printf logging / debugging function

docs/development/serial_printf_debugging.md

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+# Serial printf style debugging
+
+## Overview
+
+iNav offers a function to use serial `printf` style debugging.
+
+This provides a simple and intuitive debugging facility. This facility is only available after the serial sub-system has been initialised, which should be adequate for all but the most hard-core debugging requirements.
+
+## CLI settings
+
+It is necessary to set a serial port for serial logging using the function mask `FUNCTION_LOG`, 32768. For convenience this may be shared with MSP (mask 1), but no other function.
+For example, on a VCP port.
+
+```
+serial 20 32769 115200 115200 0 115200
+```
+
+If the port is shared, it will be resused with extant settings; if the port is not shared it is opened at 921600 baud.
+
+There are two run time settings that control the verbosity, the most verbose settings being:
+
+```
+log_level = DEBUG
+Allowed values: ERROR, WARNING, INFO, VERBOSE, DEBUG
+
+log_topics = 0
+Allowed range: 0 - 4294967295
+
+```
+
+The use of level and topics is described in the following sections.
+
+## LOG LEVELS
+
+Log levels are defined in `src/main/common/log.h`, at the time of writing these include (in ascending order):
+
+* ERROR
+* WARNING
+* INFO
+* VERBOSE
+* DEBUG
+
+These are used at both compile time and run time.
+
+At compile time, a maximum level may be defined. As of iNav 2.3, for F3 targets the maximum level is ERROR, for F4/F7 the maximum level is DEBUG.
+
+At run time, the level defines the level that will be displayed, so for a F4 or F7 target that has compile time suport for all log levels, if the CLI sets
+```
+log_level = INFO
+```
+then only `ERROR`, `WARNING` and `INFO` levels will be output.
+
+## Log Topic
+
+Log levels are defined in `src/main/common/log.h`, at the time of writing:
+
+* SYSTEM
+* GYRO
+* BARO
+* PITOT
+* PWM
+* TIMER
+* IMU
+* TEMPERATURE
+* POS_ESTIMATOR
+* VTX
+* OSD
+
+Topics are stored as masks (SYSTEM=1 ... OSD=1024) and may be used to unconditionally display log messages.
+
+If the CLI `log_topics` is non-zero, then all topics matching the mask will be displayed regardless of `log_level`. Setting `log_topics` to 4294967295 (all bits set) will display all log messages regardless of run time level (but still constrained by compile time settings), so F3 will still only display ERROR level messages.
+
+## Code usage
+
+A set of macros `LOG_S()` (log system) through `LOG_D()` (log debug) may be used, subject to compile time log level constraints. These provide `printf` style logging for a given topic.
+
+```
+//  LOG_D(topic, fmt, ...)
+LOG_D(SYSTEM, "This is %s topic debug message, value %d", "system", 42);
+```
+
+It is also possible to dump a hex representation of arbitrary  data:
+
+```
+// LOG_BUF_D(topic, buf, size)
+
+struct {...} tstruct;
+...
+LOG_BUF_D(TEMPERATURE, &tstruct, sizeof(tstruct));
+
+```
+
+## Output Support
+
+Log messages are transmitted through the `FUNCTION_LOG` serial port as MSP messages (`MSP_DEBUGMSG`). It is possible to use any serial terminal to display these messages, however it is advisable to use an application that understands `MSP_DEBUGMSG` in order to maintain readability (in a raw serial terminal the MSP message envelope may result in the display of strange characters). `MSP_DEBUGMSG` aware applications include:
+
+* msp-tool https://github.com/fiam/msp-tool
+* mwp https://github.com/stronnag/mwptools
+* iNav Configurator
+
+For example, with the final lines of `src/main/fc/fc_init.c` set to:
+
+```
+ LOG_E(SYSTEM, "Init is complete");
+ systemState |= SYSTEM_STATE_READY;
+```
+
+and the following CLI settings:
+
+```
+serial 20 32769 115200 115200 0 115200
+set log_level = DEBUG
+set log_topics = 4294967295
+```
+
+The output will be formatted as follows:
+
+```
+# msp-tool
+[DEBUG] [     3.967] Init is complete
+# mwp (stderr log file)
+2020-02-02T19:09:02+0000 DEBUG:[     3.968] Init is complete
+```
+
+The numeric value in square brackets is the FC uptime in seconds.