Auto-generate the stmhal/pybcdc_inf header file from static files

The USB VID&PID are automatically extracted from usbd_desc_cdc_msc.c
and inserted into pybcdc_inf.template, ensuring that the same USB
IDs get used everywhere
Merge pull request #498 from lurch/create-headers-from-files

Auto-generate the stmhal/pybcdc_inf header file from static files