Squashed commit of the following:

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.

drivers/syslog/Kconfig

@@ -125,6 +125,19 @@ config SYSLOG_TIMESTAMP_REALTIME
 		CLOCK_MONOTONIC, if enabled, will be used or the system timer
 		is not.
 
+config SYSLOG_PREFIX
+	bool "Prepend prefix to syslog message"
+	default n
+	---help---
+		Prepend prefix to syslog message.
+
+config SYSLOG_PREFIX_STRING
+	string "Prefix string"
+	depends on SYSLOG_PREFIX
+	default ""
+	---help---
+		The prefix string to be prepend.
+
 config SYSLOG_SERIAL_CONSOLE
 	bool
 	default n

drivers/syslog/syslog_channel.c

@@ -57,7 +57,7 @@
  * Private Function Prototypes
  ****************************************************************************/
 
-#if defined(CONFIG_SYSLOG_SERIAL_CONSOLE) && defined(CONFIG_ARCH_LOWPUTC)
+#if defined(CONFIG_ARCH_LOWPUTC)
 #  define HAVE_LOWPUTC
 #elif !defined(CONFIG_RAMLOG_SYSLOG)
 #  define NEED_LOWPUTC

drivers/syslog/syslog_consolechannel.c

@@ -54,7 +54,7 @@
  ****************************************************************************/
 
 #undef HAVE_LOWPUTC
-#if defined(CONFIG_SYSLOG_SERIAL_CONSOLE) && defined(CONFIG_ARCH_LOWPUTC)
+#if defined(CONFIG_ARCH_LOWPUTC)
 #  define HAVE_LOWPUTC 1
 #endif
 

drivers/syslog/vsyslog.c

@@ -131,13 +131,21 @@ int nx_vsyslog(int priority, FAR const IPTR char *fmt, FAR va_list *ap)
 #if defined(CONFIG_SYSLOG_TIMESTAMP)
   /* Pre-pend the message with the current time, if available */
 
-  (void)lib_sprintf(&stream.public, "[%6d.%06d]",
+  (void)lib_sprintf(&stream.public, "[%5d.%06d] ",
                     ts.tv_sec, ts.tv_nsec/1000);
 #endif
 
+#if defined(CONFIG_SYSLOG_PREFIX)
+  /* Pre-pend the prefix, if available */
+
+  ret = lib_sprintf(&stream.public, "%s", CONFIG_SYSLOG_PREFIX_STRING);
+#else
+  ret = 0;
+#endif
+
   /* Generate the output */
 
-  ret = lib_vsprintf(&stream.public, fmt, *ap);
+  ret += lib_vsprintf(&stream.public, fmt, *ap);
 
 #ifdef CONFIG_SYSLOG_BUFFER
   /* Flush and destroy the syslog stream buffer */