Fix jump from STM bootloader to application

[Semiotmo]

Jump from STM bootloader to application is currently broken.

When jumping from STM integrated bootloader to application (e.g. after a firmware update via dfu), SCB->VTOR is still set to point to the bootloaders vector table.
The STM bootloader does not reset this to the appropriate value on exit. It expects that the application itself sets the correct VTOR, as documented in AN3156, Rev 18, page 20.

The Jump to application works only if the user application sets the vector table correctly to
point to the application address.

When VTOR is not set to the applications vector table and an interrupt arrives, the application executes the interrupt handler defined in STM bootloader code instead of its own. In my case this crashed the MCU.

This pull request fixes this by making sure that the VTOR is set appropriately and no interrupt can arrive before it is set.

Additionally it ensures that after a successful jump from bootloader to application, the application does not directly jump back to bootloader by explicitly clearing the dfu_reset_to_bootloader_magic variable.
I decided to explicitly do it as well as I don't know if the STM bootloader will reset the memory associated with this variable itself.

This fix enables using the :leave parameter of dfu-util ( dfu-util -s 0x08000000:leave ...).

Closes: #195

[marckleinebudde]

Thanks for your contribution!
Works for me on stm32g0b1. Please squash suggested fix.

[marckleinebudde]

This will fix #176 (comment) and
Closes: #195