Hardcoding the root filesystem type to ext4 can't work as the default
rootfs type is squashfs. This patch fixes it by reusing the filesystem
type from ROOT_FSTYPE.

Fixes: https://git.ostc-eu.org/OSTC/OHOS/meta-ohos/-/issues/143


Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>