Add start to STM32 USB device driver.
More changes to bring STM32 USB HAL to a pretty much working state.
Clean up STM32 USB HAL
Remove magic number.
Merge branch 'master' into USBDevice-STM32F4
Revert accidental deletion
Merge pull request #119 from spectaclelabs/USBDevice-STM32F4

Add initial USB Device support for STM32F4