Config changes for PX4IO:

- enable USART3 for S.bus
 - reduce the scheduling tick to 1ms
 - disable RR scheduling
 - reduce stdio buffer sizes (not used)

nuttx/configs/px4io/io/defconfig

@@ -124,7 +124,7 @@ CONFIG_STM32_TIM7=n
 CONFIG_STM32_WWDG=n
 CONFIG_STM32_SPI2=n
 CONFIG_STM32_USART2=y
-CONFIG_STM32_USART3=n
+CONFIG_STM32_USART3=y
 CONFIG_STM32_I2C1=y
 CONFIG_STM32_I2C2=n
 CONFIG_STM32_BKP=n
@@ -341,26 +341,28 @@ CONFIG_DEBUG_CAN=n
 CONFIG_DEBUG_I2C=n
 CONFIG_DEBUG_INPUT=n
 
+CONFIG_MSEC_PER_TICK=1
 CONFIG_HAVE_CXX=n
 CONFIG_HAVE_CXXINITIALIZE=n
 CONFIG_MM_REGIONS=1
 CONFIG_MM_SMALL=y
 CONFIG_ARCH_LOWPUTC=y
-CONFIG_RR_INTERVAL=200
+CONFIG_RR_INTERVAL=0
 CONFIG_SCHED_INSTRUMENTATION=n
-CONFIG_TASK_NAME_SIZE=0
+CONFIG_TASK_NAME_SIZE=8
 CONFIG_START_YEAR=1970
 CONFIG_START_MONTH=1
 CONFIG_START_DAY=1
 CONFIG_GREGORIAN_TIME=n
 CONFIG_JULIAN_TIME=n
+# this eats ~1KiB of RAM ... work out why
 CONFIG_DEV_CONSOLE=y
 CONFIG_DEV_LOWCONSOLE=n
 CONFIG_MUTEX_TYPES=n
 CONFIG_PRIORITY_INHERITANCE=n
 CONFIG_SEM_PREALLOCHOLDERS=0
 CONFIG_SEM_NNESTPRIO=0
-CONFIG_FDCLONE_DISABLE=n
+CONFIG_FDCLONE_DISABLE=y
 CONFIG_FDCLONE_STDIO=y
 CONFIG_SDCLONE_DISABLE=y
 CONFIG_SCHED_WORKQUEUE=n
@@ -469,7 +471,7 @@ CONFIG_NPTHREAD_KEYS=4
 CONFIG_NFILE_DESCRIPTORS=8
 CONFIG_NFILE_STREAMS=0
 CONFIG_NAME_MAX=32
-CONFIG_STDIO_BUFFER_SIZE=64
+CONFIG_STDIO_BUFFER_SIZE=32
 CONFIG_STDIO_LINEBUFFER=n
 CONFIG_NUNGET_CHARS=2
 CONFIG_PREALLOC_MQ_MSGS=4