avoid reset of utility timer

[tridge]

This is broken out of the DroneCAN PR #36

resetting the UTILITY_TIMER makes accurate long term time measurement much harder. We never need to reset it if we treat it as a wrapping 16 bit unsigned number and always do timing maths with 16 bit subtraction
This means that separate parts of the code base can reliably make timing measurements as the UTILITY_TIMER is always incrementing and never reset. So we can get both timing measurements and absolute time in DroneCAN while still using the UTILITY_TIMER in other parts of the code.
It also saves a few bytes of flash, and simplifies the delay code in functions.c

todo:

• fix F421 TMR17 to be 1MHz
• check all MCUs have 1MHz UTILITY_TIMER

[freasy]

Merge when @tridge checked and tested F421

[tridge]

need to change prescaler on F421 to 119, for 120MHz periph clock

[tridge]

@AlkaMotors @freasy I've been through all the MCUs and check the prescaler against the bootloader (which relies on 1MHz for the timer). I've fixed the ones that were not 1MHz, so should be all good now