first stage mount: removing the restriction of mount points

Current first stage mount only allows three mount points: system, vendor
and/or odm. This was introduced by project Treble to mount those
verified partitions early. However, there might be some other custom
partitions needs to be mounted early as well. This CL removes the
restriction and does first stage mount for whatever specified in
fstab-dt.

Bug: 62423887
Test: first stage mount /vendor with vboot 1.0
Test: first stage mount /vendor with vboot 2.0 (AVB)

Change-Id: I6c146c64e673c35c2823523ccbde193590430c48
Merged-In: I6c146c64e673c35c2823523ccbde193590430c48
(cherry picked from commit 06ed613e)