TODO: clock_seconds has been fixed now

There were two problems:

1) clock_seconds hard-coded the divider instead of using configTICK_RATE_HZ.

2) on the FreeRTOS side, we have to explicitly set the PLL parameters
   in system_stm32f4xx.c. Only then will things work properly. I.e.,
   system_stm32f4xx.c doesn't calculate the PLL settings from the
   crystal clock as I had erroneously assumed.