Basic implementation of LPM-related functions for MSP430 MCUs
Sets MCU frequency in 'board.h', instead of the common 'cpu.h' file
Added the LPM_UNKNOWN constant to the enum 'lpm_mode'
for respresenting unknown/unavailable LPM-related status.
function 'lpm_get()' now returns LPM_UNKNOWN by default
Merge pull request #459 from rousselk/msp430-lpm-freq

Msp430 lpm freq