This commit adds some functions for sending RC5 signals.  The main
thread uses these functions to send the TV power off signal continuously
after booting.  The timings are supposed to match the RC5 standard, but
the carrier is 38 kHz because I don't know.  The TV seems to like what
I'm sending it, so I'm not worried.

Based on ChibiOS's RT-STM32F103RB-NUCLEO64 example, reconfigured for the
STM32F103C8_MINIMAL board and organized the way I like to organize my
ChibiOS projects.