Adjust the main stack of nrf51 targets to the call stack of the

softdevice.

The call stack of the soft device can be 0x600 (1536) bytes long, by
adjusting the stack to a value of 2048 bytes, their is enough room for
the softdevice and RTX kernel to live together in the main stack.

Random issues due to stack overflow were visible with the previous value.

Author: pan-

rtos/rtx/TARGET_CORTEX_M/RTX_Conf_CM.c

@@ -97,8 +97,10 @@
 #      define OS_MAINSTKSIZE    128
 #  elif defined(TARGET_STM32F334R8) || defined(TARGET_STM32F303RE) ||  defined(TARGET_STM32F303K8) ||  defined(TARGET_STM32F334C8) \
    || defined(TARGET_STM32L031K6) || defined(TARGET_STM32L053R8) || defined(TARGET_STM32L053C8) || defined(TARGET_STM32L073RZ) \
-   || defined(TARGET_EFM32HG_STK3400) || defined(TARGET_MCU_NRF51822)
+   || defined(TARGET_EFM32HG_STK3400)
 #      define OS_MAINSTKSIZE    112
+#  elif defined(TARGET_MCU_NRF51822)
+#      define OS_MAINSTKSIZE    512
 #  else
 #    error "no target defined"
 #  endif