product/rdn2: ensure feature flags are updated via sds

Validate the return value of the messaging_stack_ready function and
print error message on an error in updating the feature flags via SDS.

Signed-off-by: Nishant Sharma <nishant.sharma@arm.com>
Change-Id: Ie933aba4b30485d288321ac7df68ccee6393d3f1