Use Timer B on MSP430 architecture

Make use of TimerB where available on MSP430-based MCUs.
This allows to add (much) more available instances of hwtimers (more CCRs).
Plus, TimerB has higher interrupt priority than TimerA -- at least on families 1 and 2 of MSP430s; thus, we use it for counter overflow handling rather than TimerA.

This PR depends on #1112.