update_kernel: Use rootdev for getting the device to update the kernel

Use rootdev to get the kernel/root partition pair to update instead
of using the stateful filesystem's device.

BUG=none
TEST=./update_kernel works with an LVM stateful filesystem

Change-Id: Iad207115f9c52793f01a4dee9916bc2834304ac5
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3195048
Tested-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed