px4fmu-v1 NuttX defconfig: lower CONFIG_DEV_PIPE_SIZE to 70

This saves almost 2kb of RAM when using the mavlink shell. 70 matches the
size of the mavlink message. Since the pipe is blocking, a process writing
a lot of data will just wait, data will not be dropped.

The mavlink shell is the only process creating a pipe.

nuttx-configs/px4fmu-v1/nsh/defconfig

@@ -865,7 +865,7 @@ CONFIG_EEPROM=y
 # CONFIG_SPI_EE_25XX is not set
 CONFIG_PIPES=y
 CONFIG_DEV_PIPE_MAXSIZE=1024
-CONFIG_DEV_PIPE_SIZE=1024
+CONFIG_DEV_PIPE_SIZE=70
 CONFIG_DEV_FIFO_SIZE=0
 # CONFIG_PM is not set
 # CONFIG_POWER is not set