The ones that are moved out of iRAM should not need to be there, because
either they call functions in iROM (eg mp_hal_stdout_tx_str), or they are
only ever called from a function in iROM and not from an interrupt (eg
ets_esf_free_bufs).
This frees up about 800 bytes of iRAM.
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.
