Compatibility with newer meters

This update fixes compatibility with 34401a meters with updated front panels. It is backward compatible with older meters.

• Use firmware_release.bin if you have usb bootloader installed.
• Use firmware_debug.bin if you have usb bootloader installed and you want to see data on usb serial output. This firmware is not stable and may crash on some modes because serial is too slow for the data that the meter pushes to front panel. But it's good for checking that you have the wiring correct.
• Use firmware_nobootloader.bin if you don't have usb bootloader and want to flash firmware directly using serial dfu mode or stlink. This is same as release just linked with standard linker script that puts entry point at 0x8000000 so it works without bootloader.