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.cpp b/init/util.cpp
index 32ae93d..a101ce5 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -369,3 +369,26 @@
     os << t.duration_s() << " seconds";
     return os;
 }
+
+// Reads the content of device tree file under kAndroidDtDir directory.
+// Returns true if the read is success, false otherwise.
+bool read_android_dt_file(const std::string& sub_path, std::string* dt_content) {
+    const std::string file_name = kAndroidDtDir + sub_path;
+    if (android::base::ReadFileToString(file_name, dt_content)) {
+        if (!dt_content->empty()) {
+            dt_content->pop_back();  // Trims the trailing '\0' out.
+            return true;
+        }
+    }
+    return false;
+}
+
+bool is_android_dt_value_expected(const std::string& sub_path, const std::string& expected_content) {
+    std::string dt_content;
+    if (read_android_dt_file(sub_path, &dt_content)) {
+        if (dt_content == expected_content) {
+            return true;
+        }
+    }
+    return false;
+}