fix and clean up some board Makefiles

enables building wsn430-v1_3b and wsn430-v1_4
fixes some but not all chronos building issues
enable building chronos board
move struct timeval to msp430_types.h
Merge pull request #329 from LudwigOrtmann/make-all-the-boards

fix build for wsn430-v1_3b, wsn430-v1_4, chronos and some cleaning
use correct length for UDP checksum calculation
Merge pull request #343 from OlegHahm/udp_csum_fix

use correct length for UDP checksum calculation