stm32: Add IWDT support to ChibiOS
senoko: Save power by enabling WFI mode

WFI mode causes the idle thread to issue "Wait For Interrupt"
instruction, which cuts power consumption by about 40%.
chibios: shell: Fix backspace command

Backspace wasn't working.  It now accepts 0x7f in addition to '\b',
and prints '\b' to scribble over the line.  How both "Backspace" and ^H work.
senoko: Large commit, many changes, more rework
senoko: Break shell and watchdog out from main.c

In an effort to keep main.c from growing too large, use events, and
break shell and watchdog code into their own threads.