defconfig: sdmsteppe: disable overlayfs index

disable overlayfs index to avoid inode indexing failures when
disabling verity with dynamic partitions enabled.

Change-Id: I9d23aecc34efeec6e24c72c05cf119be4a35553e
Signed-off-by: Venkata Rao Kakani <vkakani@codeaurora.org>