It was originally in IRAM due to the linker script specification, but
since the function moved from lib/utils/interrupt_char.c to py/scheduler.c
it needs to be put back in IRAM.
The specific board can be selected with the BOARD makefile variable. This
defaults (if not specified) to BOARD=GENERIC, which is the original default
firmware build. For the 512k target use BOARD=GENERIC_512K.