cpu/lpc1768: heavy clean-up of existing code

- moved to new port structure
- rewrote startup code
- added implementation dummies for UART and timer
- switched to atmel linkerscript
- cleaned up Makefiles
board/mbed_lpc1768: heavy clean-up

- removed legacy code
- added default defines
- created periph_conf.h
Merge pull request #2045 from haukepetersen/fix_board_mbed

board/mbed_lpc1768: remodeled to our board/cpu structure