Create the build directory before running gen_usb_descriptor

[deshipu]

Otherwise it fails to create the file in that directory.

[deshipu]

When running make in a clean checkout, it fails with:

gen_usb_descriptor.py: error: argument output_file: can't open 'build-metro_m4_express/autogen_usb_descriptor.c': [Errno 2] No such file or directory: 'build-metro_m4_express/autogen_usb_descriptor.c

[tannewt]

Thank you thank you!