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/mavlink/mavlink_ftp.cpp

@@ -455,7 +455,8 @@ MavlinkFTP::_workOpen(PayloadHeader* payload, int oflag)
 	}
 	fileSize = st.st_size;
 
-	int fd = ::open(filename, oflag);
+	// Set mode to 666 incase oflag has O_CREAT
+	int fd = ::open(filename, oflag, PX4_O_MODE_666);
 	if (fd < 0) {
 		return kErrFailErrno;
 	}