init: early_mount: disallow partitions to be verified at boot

While technically possible, the verification at boot basically will
block init for as long as the entire partition is read while nothing
else is running. Disallow that as this is not going to be used anywhere.

Bug: 27805372
Test: boot angler with verifyatboot fs_mgr option for early mounted
vendor partition. That resulted in a panic() as expected.

Change-Id: I9da5caa163cae8bce6dbfb630f0ed5605ea044a0
Signed-off-by: Sandeep Patil <sspatil@google.com>