* restore bit start detection IRQ for soft serial
* restore bit start detection IRQ for softserial (target Taranis and Horus)
* target Taranis: EXTI9_5 is a shared interrupt for Rotary Encoder and Heartbeat for X7, X7-ACCESS und TPRO radios. Changed IRQ priority accordingly
* fix crashes where the heartbeat ISR running at interrupt priorities higher than 5 interferes with FreeRTOS calls made from within the heartbeat ISR.
* changed mixer scheduler timer initialization to allow for software generated update events to avoid freeRTOS calls from now prio 0 heartbeat ISR.
* resolved review feedback: using defines for IRQ prio settings
* following review by @raphaelcoeffic:
- new function void mixerSchedulerSoftTrigger() (to be called by heartbeat driver)
- defines in hal.h for targets horus and taranis
* following @raphaelcoeffics review
- changed all but telemetry EXIT IRQ (soft serial) definition back to their original state and added define to override EXIT IRQ (soft serial) prio
