This board has only 2MiB of flash so the build needs to be reduced in size
to fit. Commit 549448e8bbc8ce0b6b5fc51c0660acdaff18c3d6 made all boards
build with -O2 by default (for performance) so this overrides that default.
Signed-off-by: Damien George <damien@micropython.org>