arduino-hal: Add support for Arduino Micro

[Ivan-Johnson]

@Rahix, I've got a couple of questions:

1. Should we define default_serial?

   Given the fact that the ATmega32U4 doesn't have a hardware serial, my gut reaction is that we don't need it. However, both of the existing ATmega32U4 boards do define default_serial. Am I incorrect in assuming that default_serial is used for the default hardware serial bus?

2. Other than verifying that the new arduino-micro examples work (they do), are there any other tests that you'd like me to run?

   In particular: when I try running cargo build from the root directory, I get a ton of build errors. I get the same errors even without this PR applied (build_logs.txt). Is this an issue with my build environment, or is cargo build only expected to work when run from within an example package?

[Rahix]

Thanks for picking this up!

Should we define default_serial?

Yeah, I think we still should do it. You can connect a separate USB-UART bridge to the RX/TX pins to talk to the board so it is nice to have the short-hand macro for setting up communication.

Other than verifying that the new arduino-micro examples work (they do), are there any other tests that you'd like me to run?

No, this is entirely fine. Running cargo build from the crate root is expected to fail, you must run it from inside each directory. But in the end, just let the CI run those build tests for you ;)

[Ivan-Johnson]

You can connect a separate USB-UART bridge to the RX/TX pins to talk to the board

Ah, that makes sense; I didn't realize until now that UART only works on those two pins in particular.

I've pushed an update that enabled default_serial. I don't have easy access to a USB-UART bridge of my own, but I have created an micro-usart.rs so that we can at least verify that it compiles successfully. I assume that's good enough; as best I can tell the Arduino Micro and Arduino Leonardo hardware boards are essentially identical aside from their physical sizes

[Rahix]

Thanks a lot! :)