The checksum calculation process was causing the CPLD configuration to reload from flash. With the new SRAM loading mechanism, flash contents may not be up to date, so the CPLD bitstream may regress to the point of not working or not working correctly. This commit is a short-term fix for mossmann/hackrf issue #609.
Among the TODOs:
* Refactor obtaining LPC serial number and chip ID into separate API and header/source files. Remove from main().
* Create a usb_set_serial_number_descriptor() or similar function to be called before USB stack is started.
* Ensure USB serial number descriptor is valid even if code forgets to initialize the serial number before the USB stack is started. May be as simple as providing default initializer for usb_descriptor_string_serial_number[].
* Create a #define/constant for the usb_descriptor_string_serial_number length.
* Identify what's causing intermittent crashes in hackrf_transfer when no serial number is specified. I'm probably misusing getopt.
* Permit serial number without leading zeros so you don't have to type as much.
* Add support for serial number argument in other hackrf_* tools.
* Provide libhackrf support for enumerating multiple HackRFs, so that hackrf_info can list all devices. May require an additional libhackrf function, outside of hackrf_open().
...and anything else that makes this less of a hack.
