fsmgr: allow for a multiple fs-types for a mount point.

This changes allows an fstab to contain multiple lines for a given
mount point.
The lines sharing a mount MUST be after each other.

The 1st matching line is the primary when it comes to mounting
and look ups for wiping.

Mounting based on a mount_point will attempt each dup in turn
until one succeeds.

This is to allow quick experimentations between different FSes.
It does not deal with checkfs yet, because the underlying invocation
of fs-type appropriate fsck does not handle the error code.
Only the primary FS (1st in the dups) is checked.

Change-Id: I8329737454b53e2681436fe85cd00a9bc522676b
Signed-off-by: JP Abgrall <jpa@google.com>