Have logging on zephyr use sys_log mechanism

[galak]

We currently use printk, but should tie into the proper logging system in zephyr via include/logging/sys_log.h.

[skordal]

This won't directly work with the current libmetal logging API. The problem is that the Zephyr syslog APIs are macros (SYS_LOG_ERR()/SYS_LOG_WRN/etc), so we can't call them correctly from the libmetal log handler.

However, we could call the SYS_LOG_BACKEND_FN instead of printk from the log handler. We won't get color support or the consistent logging format provided by the SYS_LOG_ macros, but we can support custom Zephyr logging functions (syslog_hook).

[galak]

I'll add a comment to zephyrproject-rtos/zephyr#6186 to improve the logging API in zephyr to allow us to connect this more directly.

[galak]

@skordal can we use or tie into LOG_NO_COLOR()?

[skordal]

That won't work either, we will need a function that can take a va_list argument, since we need to forward the arguments passed to the libmetal log handler function.

(My suggestion about using the SYS_LOG_BACKEND_FN actually won't work either, for the same reason).