Add semihosting to allow debug without USB/serial

low quality personal notes follow - will revise before PR

Semihost

investigate semihosting for talking to python client
Automatically change serial port to use structured log messages ocne it hears from python client
Use a reboot code register as a 'mailbox' to communicate that the ICE wants semihosting used by the target. https://devzone.nordicsemi.com/f/nordic-q-a/13631/nrf52-resetreas-register https://infocenter.nordicsemi.com/topic/ps_nrf52840/power.html?cp=5_0_0_4_2_6_10#register.RESETREAS
FIXME - support 'printing' debug output to the pyocd "semihost" server. https://pyocd.io/docs/semihosting.html and possibly this ardiuno lib https://registry.platformio.org/libraries/koendv/STM32duino-Semihosting https://docs.platformio.org/en/latest/advanced/unit-testing/semihosting.html