Improved logging with both compile and runtime level filtering

The device level debug will have to be removed and the debugging
can be based on this new logging structure which can tell where
an error (or debug output) occured whch the current implmentation
cannot.

The one limitation is the new macros cannot take a char* for the
format parameter. It must be an actual string literal because it
is concatenated with other strings.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>

src/platforms/common/module.mk

@@ -2,5 +2,6 @@
 # Common OS porting APIs
 #
 
-SRCS		 = px4_getopt.c
+SRCS		 = px4_getopt.c \
+		   px4_log.c
 

src/platforms/common/px4_log.c

@@ -0,0 +1,5 @@
+#include <px4_log.h>
+
+__EXPORT unsigned int __px4_log_level_current = PX4_LOG_LEVEL_AT_RUN_TIME;
+
+__EXPORT const char *__px4_log_level_str[_PX4_LOG_LEVEL_DEBUG+1] = { "INFO", "PANIC", "ERROR", "WARN", "DEBUG" };