Documentation: Add description for early_syslog()

Add documentation describing the purpose and behavior of early_syslog(),
   which provides a minimal logging facility available during early system
   initialization or system down.

Signed-off-by: Chengdong Wang wangchengdong@lixiang.com

Documentation/components/drivers/special/syslog.rst

@@ -67,6 +67,26 @@ The above are all standard interfaces as defined at
 Those interfaces are available for use by application software.
 The remaining interfaces discussed in this section are non-standard, OS-internal interfaces.
 
+Early Syslog Interfaces
+-----------------------
+
+Provides a minimal SYSLOG output facility that can be used during the
+very early boot phase or when the system is in a down state, before the
+full SYSLOG subsystem or scheduler becomes available.
+
+.. c::function:: void early_syslog(FAR const char *fmt, ...);
+
+  See ``include/nuttx/syslog/syslog.h``.
+  This function provides basic formatted output similar to printf(),
+  and writes the resulting characters directly to the low-level output
+  device via up_putc(). It is primarily intended for debugging or
+  diagnostic messages in early system contexts, where interrupts may
+  be disabled and locking mechanisms are not yet available.
+
+  The function automatically appends a newline character ('\n') if
+  the formatted message does not already end with one, ensuring proper
+  alignment of log output in serial consoles or early boot traces.
+
 Debug Interfaces
 ----------------
 

drivers/syslog/Make.defs

@@ -23,7 +23,7 @@
 ############################################################################
 # Include SYSLOG Infrastructure
 
-if(CONFIG_ARCH_LOWPUTC)
+ifeq ($(CONFIG_ARCH_LOWPUTC),y)
   CSRCS += syslog_early.c
 endif