stmhal: Improve efficiency of SysTick IRQ and HAL_Delay.
SysTick IRQ now increases millisecond counter directly (ie without calling HAL_IncTick). Provide our own version of HAL_Delay that does a wfi while waiting. This more than halves power consumption when running a loop containing a pyb.delay call. It used to be like this, but new version of HAL library regressed this feature.