STM32F1 STM32VLDicovery board: incorrect clock register setting
Description of problem:
The execution of the program hangs when testing, from libopencm3 clock initialization, the status of the clock in ``rcc_wait_for_osc_ready()``. This function https://github.com/libopencm3/libopencm3/blob/master/lib/stm32/f1/rcc.c#L366 loops until the bit stating that the oscillator is stabilized is set. I am unable to find in qemu this bit being set upon clock initialization, which I believe is an hardware emulation shortcoming. Commenting this line in libopencm3 allows for the emulation to complete correctly, but I believe the error lies in the hardware emulation and not in the libopencm3 test. Reading the status of ``RCC_CR`` from ``gdb-multiarch`` probing the QEMU internal state returns 0, leading to the failure of the test at https://github.com/libopencm3/libopencm3/blob/master/lib/stm32/f1/rcc.c#L353

See https://www.st.com/resource/en/reference_manual/rm0008-stm32f101xx-stm32f102xx-stm32f103xx-stm32f105xx-and-stm32f107xx-advanced-armbased-32bit-mcus-stmicroelectronics.pdf for the expected behavior of this register content on page 99/1136
```
7.3.1 Clock control register (RCC_CR)

Bit 17 HSERDY: External high-speed clock ready flag
Set by hardware to indicate that the HSE oscillator is stable. This bit needs 6 cycles of the
HSE oscillator clock to fall down after HSEON reset.
0: HSE oscillator not ready
1: HSE oscillator ready
```
Steps to reproduce:
1. git clone --recursive https://github.com/libopencm3/libopencm3-examples
2. make
3. qemu-system-arm -M stm32vldiscovery -nographic -serial mon:stdio -kernel examples/stm32/f1/stm32vl-discovery/usart/usart.elf