init: moving early mount logic into init_first_stage.cpp

Also renames "early mount" to "first stage mount" to prevent confusion
with "mount_all --early", which is run in the init second stage.

Also creates a base class: FirstStageMount and two derived classes:
FirstStageMountVBootV1 and FirstStageMountVBootV2 to replace/refactor
existing functions:

   - early_mount() -> DoFirstStageMount() and FirstStageMount::DoFirstStageMount()

   - vboot_1_0_early_partitions -> FirstStageMountVBootV1::GetRequiredDevices()
   - vboot_2_0_early_partitions -> FirstStageMountVBootV2::GetRequiredDevices()

   - vboot_1_0_mount_partitions ->
       FirstStageMount::MountPartitions() and
       FirstStageMountVBootV1::SetUpDmVerity()

   - vboot_2_0_mount_partitions ->
       FirstStageMount::MountPartitions() and
       FirstStageMountVBootV2::SetUpDmVerity()

Bug: 37413399
Test: first stage mount /vendor with vboot 2.0 (avb) on bullhead
Test: first stage mount /system with without verity on bullhead
Test: first stage mount /vendor with with vboot 1.0 on sailfish
Change-Id: I6584bdf7d832c9fbc8740f97c9b8b94e68a90783