add option to customize stm32 time driver MIN_STOP_PAUSE via a env variable

[dragonnn]

On my STM32F401RET6 for some reason the default 250ms MIN_STOP_PAUSE is really unstable, often timed wakeups or wakeups from EXTI GPIO are missed, they is some trace output when those happen but it isn't passed to the task waiting for those. Changing the MIN_STOP_PAUSE to 2500ms complete fixes those issues. I know that isn't a solution but more kind of workaround but that is how far I currently got and for my application that is a completely fine trade off. And I think having the ability to adjust that value might be for other user usable too.