ivory-bsp-stm32f4: create BoardHSE class to give hseFreq

* as I said in the old code itself - the frequency isn't a constant!
  but it was hard coded like one anyway
* source of weird baud rate problems on open407 board I saw
  over the weekend.
ivory-bsp-stm32f4: spi, uart library code uses BoardHSE
ivory-bsp-stm32f4: test apps use BoardHSE
ivory-px4-hw: test apps use BoardHSE
ivory-bsp-stm32f4: revisions to TIM2, TIM5 types

* usable as both 16 and 32 bit timers, so it can
  be more generic to just consider them as 16
flight: uses BoardHSE