Added support for Nucleo F303RE

I am not sure if it's the right SP, but the basic blink code works.
What does the SP stand for? Stack pointer?

Also, if you could tell me where to look for the correct address, I would.
I looked in the reference manual and the programming manual that STM provides, with no luck.

Cheers

Author: dominus97

libraries/rtos/rtx/TARGET_CORTEX_M/RTX_CM_lib.h

@@ -298,6 +298,9 @@ osThreadDef_t os_thread_def_main = {(os_pthread)main, osPriorityNormal, 0, NULL}
 #elif defined(TARGET_STM32F401VC)
 #define INITIAL_SP            (0x20010000UL)
 
+#elif defined(TARGET_STM32F303RE)
+#define INITIAL_SP            (0x20010000UL)  //Not sure if right address, but it works!
+
 #else
 #error "no target defined"