Push single wire operations into stm32_serial.c

I have moved the STM32 serial calls into stm32_serial that will allow for enabling and disabling of single wire mode only.

To verify and illustrate how it is called I setup a test (tests hott_telemetry), which not only tests enabling/disabling single wire mode but tests HoTT telemetry in general.  Just having a test for single wire didn't seem to justify a test class and it will be useful once HoTT is added into the codebase.