log: Add ability to add per-file log prefix

If LOG_PREFIX is defined before including log.h then all pr_xxx()
functions prepend message with (LOG_PREFIX ": "). pr_raw() prints
a message without prefix.

Change-Id: I47b970717a0038b37cce46591f6f085a3ccb5e4c

Author: bunder

log.c

@@ -14,7 +14,7 @@
 
 enum loglevel vlevel = LOG_WARNING;
 
-void pr_level(enum loglevel const level, char const *format, ...)
+void pr_raw(enum loglevel const level, char const *format, ...)
 {
 	if (level <= vlevel) {
 		FILE *const stream = level < LOG_INFO ? stderr : stdout;

log.h

@@ -20,9 +20,15 @@ enum loglevel {
 
 extern enum loglevel vlevel;
 
-void pr_level(enum loglevel const level, char const *format, ...);
+void pr_raw(enum loglevel const level, char const *format, ...);
 void pr_cont(enum loglevel const level, char const *format, ...);
 
+#ifdef LOG_PREFIX
+#define pr_level(level, format, ...) pr_raw(level, LOG_PREFIX ": " format, ##__VA_ARGS__)
+#else
+#define pr_level(level, format, ...) pr_raw(level, format, ##__VA_ARGS__)
+#endif
+
 #define pr_err(format, ...)   pr_level(LOG_ERROR, format, ##__VA_ARGS__)
 #define pr_warn(format, ...)  pr_level(LOG_WARNING, format, ##__VA_ARGS__)
 #define pr_info(format, ...)  pr_level(LOG_INFO, format, ##__VA_ARGS__)