custom-sn-v1.0-rc1

Added support for custom USB Serial Number

Declared external char g_usbSerialNumber[33] - 32 characters and '\0'.
Checking if it is not empty (!= '\0') then assign the value to name variable which will be used as Serial Number.
If it is empty then fallback to MCU UID happens.
All changes are considered only when there will be specified build flag USB_CUSTOM_SERIAL.

In PlatformIO add build flag to platformio.ini:

build_flags = -DUSB_CUSTOM_SERIAL

For Arduino IDE support changes in boards.txt were made.
Added a new menu to choose in Tools menu: Custom Serial with option to Enable or Disable (default).
Enable it to add build flag USB_CUSTOM_SERIAL.