The signature is provided by the bootloader, so you can configure the bootloader to lie about the signature in order to allow uploading to the board with the "Arduino Nano" board definition selected from Arduino IDE's Tools > Board menu without a signature verification error.
https://github.com/Optiboot/optiboot/issues/184#issuecomment-230144696
the optiboot bootloader returns a signature that is hard-coded in the bootloader, rather than the actual signature of the chip. ie - the bootloader lies