Test flight has been performed with nonlinear SO(3) attitude estimator.

Here are few observations:
 - When the system initialized, roll angle is initially reversed.
   As filter converged, it becomes normal.
 - I put a negative sign on roll, yaw. It should naturally has right
   sign, but I do not know why for now. Let me investigate again.
 - Gain : I do not know what gain is good for quadrotor flight.
   Let me take a look Ardupilot gain in the later.

Anyway, you can fly with this attitude estimator.

nuttx/configs/px4fmu/nsh/defconfig

@@ -248,7 +248,7 @@ CONFIG_SERIAL_TERMIOS=y
 CONFIG_SERIAL_CONSOLE_REINIT=y
 CONFIG_STANDARD_SERIAL=y
 
-CONFIG_USART1_SERIAL_CONSOLE=y
+CONFIG_USART1_SERIAL_CONSOLE=n
 CONFIG_USART2_SERIAL_CONSOLE=n
 CONFIG_USART3_SERIAL_CONSOLE=n
 CONFIG_UART4_SERIAL_CONSOLE=n
@@ -561,7 +561,7 @@ CONFIG_START_MONTH=1
 CONFIG_START_DAY=1
 CONFIG_GREGORIAN_TIME=n
 CONFIG_JULIAN_TIME=n
-CONFIG_DEV_CONSOLE=y
+CONFIG_DEV_CONSOLE=n
 CONFIG_DEV_LOWCONSOLE=n
 CONFIG_MUTEX_TYPES=n
 CONFIG_PRIORITY_INHERITANCE=y
@@ -925,7 +925,7 @@ CONFIG_USBDEV_TRACE_NRECORDS=512
 #   Size of the serial receive/transmit buffers. Default 256.
 #
 CONFIG_CDCACM=y
-CONFIG_CDCACM_CONSOLE=n
+CONFIG_CDCACM_CONSOLE=y
 #CONFIG_CDCACM_EP0MAXPACKET
 CONFIG_CDCACM_EPINTIN=1
 #CONFIG_CDCACM_EPINTIN_FSSIZE