make: provide BOARD, MCU and CPU name as a string

This PR adds the defined `RIOT_BOARD`, `RIOT_CPU` and `RIOT_MCU`.
They each are defined as `BOARD_${BOARD}`, `CPU_${CPU}`, and
`MCU_${CPU}`, resp.

`BOARD_${BOARD}` is defined as the string `"${BOARD}"`, but not in upper
case. Same for the other two.