Allow target or application level definition of extra TICKLESS stack size

Adding a check to let application or target force increase idle thread
stack size.

Author: LMESTM

rtos/TARGET_CORTEX/mbed_rtx_conf.h

@@ -40,7 +40,7 @@
 #endif
 
 // Increase the idle thread stack size when tickless is enabled
-#if defined(MBED_TICKLESS) && defined(LPTICKER_DELAY_TICKS) && (LPTICKER_DELAY_TICKS > 0)
+#if defined(EXTRA_IDLE_STACK_REQUIRED) || (defined(MBED_TICKLESS) && defined(LPTICKER_DELAY_TICKS) && (LPTICKER_DELAY_TICKS > 0))
 #define EXTRA_IDLE_STACK MBED_CONF_RTOS_IDLE_THREAD_STACK_SIZE_TICKLESS_EXTRA
 #else
 #define EXTRA_IDLE_STACK 0

rtos/mbed_lib.json

@@ -19,7 +19,7 @@
             "value": 4096
          },
          "idle-thread-stack-size-tickless-extra": {
-            "help": "Additional size to add to the idle thread when tickless is enabled and LPTICKER_DELAY_TICKS is used",
+            "help": "Additional size to add to the idle thread when a specific target or application implementation requires it or in case tickless is enabled and LPTICKER_DELAY_TICKS is used",
             "value": 256
          },
          "idle-thread-stack-size-debug-extra": {