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
board - pttu: added periph_conf.h
Merge pull request #1318 from haukepetersen/msba2_pwm

cpu: added low-level PWM driver for the lpc2387