WIP: Add BBC micro:bit board

[aykevl]

Not yet finished: the stm32 target breaks with the changes to .svd file parsing. But the micro:bit works! I've tested examples/serial.

closes #27

[deadprogram]

So some things not yet working on Microbit. Specifically, the IRQ handler for the UART is not getting triggered.

https://github.com/aykevl/tinygo/blob/1dfe3600938b1b5bbfa536c25003a87a5692aa94/src/machine/machine_nrf.go#L95

I would have expected to change this to then work //go:export UART0_IRQHandler but it does not get triggered.

[aykevl]

You can take a look at src/device/nrf/nrf51.s for the proper interrupt name.

[aykevl]

@deadprogram can you maybe test whether I2C works, on the nrf52 and the nrf51?

This PR is of course not finished but I wanted to share what I have so far.

[deadprogram]

src/device/nrf/nrf51.s has an entry IRQ UART0_IRQHandler which is where I came up with //go:export UART0_IRQHandler.

[aykevl]

I fixed a few things in the last commits and I think it is ready now.
@deadprogram what do you think? Does it work for you?
I changed some I2C code so would like to make sure I didn't break anything (for the nrf52).

[deadprogram]

OK just tested UART on both pca10040 and on Microbit. Both worked as expected.

Then tested I2C on pca10040 and worked as previous. I do not currently have the correct adaptor for the Microbit edge connector to test i2c on that with my BlinkM.

Seems like ready to merge. I will submit a PR with some code for the correct button mappings and also some basic code for the LED matrix once it lands.

[aykevl]

Thank you for testing!

Merged in bc9210b.