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
Introduced a cleaned-up cpu/core interface

- Included a collection of cpu-dependent headers in core/include/arch
- Extracted all interfaces that need to be implemented for a cpu
- Created a mapping between those interfaces and the old ones

Conflicts:
	core/include/lpm.h
core: added flag for disabling arch interface
core: added missing state to lpm_arch interface