drivers/syslog: Channel configuration. Decouple SYSLOG_SERIAL_CONSOLE and ARCH_LOWPUTC. Since some hardware can output log to the special debug channel not serial.
drivers/syslog/vsyslog.c: Support pre-pending a prefix string to log output if enabled. This very useful to identify which cpu send out the log in AMP SoC.
drivers/syslog/vsyslog.c: Make timestamp output same as linux kernel. It's very useful if NuttX syslog retarget to Linux syslog.