console: Implement CONFIG_CONSOLE_FROM_ISR for libopencm3

Modify _write() to make it safer to use stdio from interrupt (this simply
avoids locking up in the ISR... it does not make it safe to use stdio
from multiple calling contexts).

After that running from ISR is simply a matter of replacing
console_putchar() with console_process().

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>