qemu-system-arm freezes when using SystickTimer on netduinoplus2

git commit 27c94566379069fb8930bb1433dcffbf7df3203d

The global variable system_clock_scale used in hw/timer/armv7m_systick.c is never set on the netduinoplus2 platform, it stays initialized as zero. Using the timer with the clock source as cpu clock leads to an infinit loop because systick_timer->tick always stays the same.

To reproduce use to CMSIS function SysTick_Config(uint32_t ticks) to setup the timer.