cpu:cortex_common: add core_panic function
Merge pull request #1313 from thomaseichinger/cortexm_crash

cpu:cortex_common: add core_panic function
cpu: added low-level pwm driver impl for lpc2387
boards: added periph_conf.h for msba2 and avsextrem
drivers: some adjustments to low-level pwm driver

- adjusted doxygen and boilerplate
- changed return values for start and stop functions