[sys_time] some performance improvements
stm32 now uses AHB_CLK / 8 as clock input for systick
[sys_time] cleanup and get_sys_time_float() inline function
get_sys_time_usec for stm32, overflows after 70min (same as before)
currently only used in SysTimeTimer, which is only used at startup AND
the sys_mon module, so that module doesn't work anymore after 70min!
[sys_time] cleanup: use sys_time.nb_x instead of old cpu_time_x