init: Don't fail first stage mount for formattable partitions.

If a partition can be formatted by vold, then it doesn't make sense to
have first-stage mount fail if the partition hasn't been formatted yet.

Bug: 121209697
Test: device boots after fastboot flashall -w
Change-Id: I4a6c794a9912a5e0794983e9514a08f9c23e6ae9