Use correct prototype of benchmark_timer_read()

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.

c/src/lib/libbsp/lm32/shared/milkymist_timer/timer.c

@@ -47,7 +47,7 @@ void benchmark_timer_initialize(void)
                              /* This value is in microseconds. */
 #define LEAST_VALID       4  /* Don't trust a clicks value lower than this */
 
-uint32_t benchmark_timer_read(void)
+benchmark_timer_t benchmark_timer_read(void)
 {
   uint32_t ticks;
   uint32_t total;

c/src/lib/libbsp/lm32/shared/timer/timer.c

@@ -76,7 +76,7 @@ void benchmark_timer_initialize( void )
                              /* This value is in microseconds. */
 #define LEAST_VALID       4  /* Don't trust a clicks value lower than this */
 
-uint32_t benchmark_timer_read( void )
+benchmark_timer_t benchmark_timer_read( void )
 {
   uint32_t ticks;
   uint32_t total;