[Nuvoton] Fix idle thread stack underflow in debug build

In rtos/mbed_lib.json:
Enlarge idle-thread-stack-size-debug-extra for all Nuvoton targets.

In rtos/TARGET_CORTEX/mbed_rtx_conf.h:
MBED_DEBUG also must be defined for this configuration parameter to take effect.

rtos/mbed_lib.json

@@ -49,6 +49,9 @@
 	    },
 	    "MCU_NRF51_UNIFIED": {
 	        "target.boot-stack-size": "0x800"
+	    },
+        "NUVOTON": {
+            "idle-thread-stack-size-debug-extra": 512
         }
     }
 }