[RFC] Add new SYS_LOG_BACKEND_FN to sys_log

Perry, Eitan <eitan.perry@...>

Add new SYS_LOG_BACKEND_FN to sys_log.h
Current system log implementation:
* When using system log API (SYS_LOG_ERR(...), SYS_LOG_WRN(...),
SYS_LOG_INF(...), SYS_LOG_DBG(...) ), each of the four SYS_LOG_X macros is
calling the same macro - SYS_LOG_BACKEND_FN that should output the log
* SYS_LOG_BACKEND_FN is mapped to printk or printf according to kconfig.

* Have the ability to use various back ends to output the messages,
i.e. different UART than the console, SPI, flash, FS, etc..
* The users should use the log in asynchronous way (i.e. not wait to IO

* Add the option to map SYS_LOG_BACKEND_FN to a new function, that will
Put the log message in a cyclic_buffer, depends on kconfig.
* Keep the same system log api.
* The log messages will be written to a cyclic buffer (instead of
printk / printf).
* Application developers may add various back ends, that will read the
messages from the cyclic buffer and output the data to the required

Code change suggestion
At: /include/misc/sys_log.h

/* decide print func */
#include <bsp_log_cyclic_buffer.h>
#define SYS_LOG_BACKEND_FN log_cyclic_buf_put
#include <stdio.h>
#define SYS_LOG_BACKEND_FN printf
#include <misc/printk.h>
#define SYS_LOG_BACKEND_FN printk

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