cpu/arm/k60: less rtimer drift.

 - Added compensation for drift caused by preempting of the interrupt
   handler and function prologue and code within the interrupt handler.
 - Move LDVAL store to after rtimer_arch_now() call in ISR to get the
   correct offset computation when dealing with scheduled rtimer tasks.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
examples/lis3dh-fifo: printf changes.

 - use \n instead of \r\n for line endings.
 - Added an experimental call to float printf as a test.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: llwu_init cleanup.

 - BITBAND_REG for clock gate.
 - Remove TODO about symbolic NVIC IRQ names, fixed since CMSIS refactor.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>