This change starts with removing the effectively empty file
timerdrv.h. The prototypes for benchmark_timer_XXX() were in
btimer.h which was not universally used. Thus every use of
timerdrv.h had to be changed to btimer.h. Then the prototypes
for benchmark_timer_read() had to be adjusted to return
benchmark_timer_t rather than int or uint32_t.
I took this opportunity to also correct the file headers to
separate the copyright from the file description comments which
is needed to ensure the copyright isn't propagated into Doxygen
output.
