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
diff --git a/init/util.h b/init/util.h
index 0bb9cdf..92b3a1d 100644
--- a/init/util.h
+++ b/init/util.h
@@ -29,6 +29,8 @@
 
 #define COLDBOOT_DONE "/dev/.coldboot_done"
 
+const std::string kAndroidDtDir("/proc/device-tree/firmware/android/");
+
 using android::base::boot_clock;
 using namespace std::chrono_literals;
 
@@ -72,4 +74,8 @@
 
 void panic() __attribute__((__noreturn__));
 
+// Reads or compares the content of device tree file under kAndroidDtDir directory.
+bool read_android_dt_file(const std::string& sub_path, std::string* dt_content);
+bool is_android_dt_value_expected(const std::string& sub_path, const std::string& expected_content);
+
 #endif