Fixed passed ot open() for O_CREAT

In nuttx the mode parameter to open is not required but in Linux,
and per the POSIX spec, mode is required if the O_CREAT flag is
passed.

The mode flags are different for NuttX and Linux so a new set of
PX4 defines was added:

PX4_O_MODE_777 - read, write, execute for user, group and other
PX4_O_MODE_666 - read, and write for user, group and other
PX4_O_MODE_600 - read, and write for user

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

src/modules/sdlog2/sdlog2.c

@@ -505,7 +505,7 @@ int open_log_file()
 		}
 	}
 
-	int fd = open(log_file_path, O_CREAT | O_WRONLY | O_DSYNC, 0x0777);
+	int fd = open(log_file_path, O_CREAT | O_WRONLY | O_DSYNC, PX4_O_MODE_666);
 
 	if (fd < 0) {
 		mavlink_and_console_log_critical(mavlink_fd, "[sdlog2] failed opening: %s", log_file_name);