Commit 0abf6f830c removed _boot.py from the
manifest for the GENERIC_512K board because the build does not include a
filesystem. But the main code expects _boot.py to be there and prints an
error if it's not. So add a custom _boot.py, which just sets the
gc.threshold().
Signed-off-by: Damien George <damien@micropython.org>
