cpu: added enableIRG implementation to cpu/lpc1768
hwtimer: clean up of interfaces to board/cpu
Merge pull request #919 from haukepetersen/hwtimer_cleanup

core/cpu: renamed ARCH_MAXTIMERS to HWTIMER_MAXTIMERS
Merge pull request #923 from haukepetersen/mbed_enableIRQ

cpu: added enableIRQ implementation to cpu/lpc1768