Factor out micro:bit LED roulette into turn-key example

[sirhcel]

• Let's lower the bar four young players and have a working reference right out of the repository
• I could be put into action with cargo-flash with one of the following invocations:

  $ cargo flash --example my-solution --features v2 --target thumbv7em-none-eabihf --chip nrf52833_xxAA
  $ cargo flash --example my-solution --features v1 --target thumbv7em-none-eabihf --chip nrf51822_xxAA

[bors[bot]]

Build succeeded:

• build-book
• build-f3discovery-chapter (05-led-roulette)
• build-f3discovery-chapter (06-hello-world)
• build-f3discovery-chapter (07-registers)
• build-f3discovery-chapter (08-leds-again)
• build-f3discovery-chapter (09-clocks-and-timers)
• build-f3discovery-chapter (11-usart)
• build-f3discovery-chapter (14-i2c)
• build-f3discovery-chapter (15-led-compass)
• build-f3discovery-chapter (16-punch-o-meter)
• build-f3discovery-doc
• build-microbit-chapter (05-led-roulette)
• build-microbit-chapter (07-uart)
• build-microbit-chapter (08-i2c)
• build-microbit-chapter (09-led-compass)
• build-microbit-chapter (10-punch-o-meter)
• build-microbit-doc