Use HWSERIAL1 to detect HW serial mode

[Jawell]

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

rdm6300/examples/hardware_uart/hardware_uart.ino

Line 5 in a8ddb5f

* Will only compile for boards with Serial1 or more:

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

[Jawell]

@arduino12 ping :)