19706: boards/olimex-msp430-h2618: add new board r=maribu a=maribu
### Contribution description
This adds support of the Olimex MSP430-H2618 board. This board has plenty of RAM and ROM for an MSP430 board (even more than the z1), but still is easily obtainable.
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@ovgu.de>
Co-authored-by: Marian Buschsieweke <marian.buschsieweke@posteo.net>