Use HWSERIAL1 to detect HW serial mode

[Jawell]

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Hi! I have Nano Every and I want to use HW serial, but there is no directive for this. I thought to add ARDUINO_AVR_NANO_EVERY, but HW serial must be complied only for boards with Serial1 or more, as you mentioned in comment https://github.com/arduino12/rdm6300/blob/a8ddb5fb153177d049141c1307f9e542f49bbaa3/examples/hardware_uart/hardware_uart.ino#L5

So, I added HWSERIAL1 to check this and avoid describing board list. What do you think?

[Jawell]

@arduino12 ping :)