init: allow first stage mount to read an fstab

Now that we have a first stage ramdisk (or use recovery in its place),
it's possible to place a vendor specific fstab along with first stage
init, removing the need for device tree modifications to have an
fstab.

Bug: 117933812
Test: hikey boots with only an fstab in first stage ramdisk
Test: blueline mainline boots with a disabled DT fstab and an fstab in recovery
Change-Id: I4460b88851557a75ba06ff795cd842e7dfb6da46