Systick Interrupt should never be activated using NVIC_EnableIRQ because it's an system exception (#850)

ports/stm32f10x/mstimer-init.c

@@ -48,5 +48,4 @@ void mstimer_init(void)
             /* config error? return  1 = failed, 0 = successful */
         }
     }
-    NVIC_EnableIRQ(SysTick_IRQn);
 }

ports/stm32f4xx/mstimer-init.c

@@ -50,5 +50,4 @@ void mstimer_init(void)
             /* config error? return  1 = failed, 0 = successful */
         }
     }
-    NVIC_EnableIRQ(SysTick_IRQn);
 }