lpc1768: cleanup Makefile

`cpu/lpc1768/Makefile` contains many lines that don't belong into this
file, but the maybe `Makefile.include`. Either way, this Makefile is
never called with these goals.
cpu - sam3x8e: fixed vtimer, added timer_set_absolute
Merge pull request #1358 from haukepetersen/fix_sam3x_vtimer

cpu - sam3x8e: fixed vtimer, added timer_set_absolute
Merge pull request #1187 from Kijewski/mbed_lpc1768-makefile-cleanup

lpc1768: cleanup Makefile