With oe-core/65498411d73e8008d5550c2d0a1148f990717587
creating the image manifest for FreeRTOS raises an
exception. FreeRTOS support is experimental and there
is an issue in rootfs generation, so we disable the option
for now.

Signed-off-by: Davide Gardenal <davide.gardenal@huawei.com>