uboot-imx

api: Convert to Kconfig

Now that we have a Kconfig for the API, convert the two boards that
are using this to Kconfig and remove CONFIG_API from the whitelist.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>