bch: Allow to build for the host

We will need the bch functions in the tool to generate the SPL images for
the Allwinner SoCs.

Do the needed adjustments so that we can use it on the host.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>