Corrected main thread stack size, was accidently updated by removing condition

Main thread stack size is reduced for TARGET_STM32F070RB and TARGET_STM32F072RB,
in recent update to mbed_rtx.h the conditional check for main thread size update
was removed.

Author: Deepika

targets/TARGET_STM/mbed_rtx.h

@@ -130,9 +130,11 @@
 
 #endif // INITIAL_SP
 
+#if (defined(TARGET_STM32F070RB) || defined(TARGET_STM32F072RB))
 #ifdef MBED_CONF_RTOS_MAIN_THREAD_STACK_SIZE
 #undef MBED_CONF_RTOS_MAIN_THREAD_STACK_SIZE
 #endif
 #define MBED_CONF_RTOS_MAIN_THREAD_STACK_SIZE 3072
+#endif
 
 #endif  // MBED_MBED_RTX_H