Add update_bootloader_message() to fix two-step OTAs.

This is a retry of commit 7e31f421a514da09b90e46dbd642a5e9b16e0003.

Commit bd56f1590c967205dc45eb2ec298aa8d2aacb740 switches to calling
write_bootloader_message(<options>) in get_args(), which
unintentionally resets the stage field thus breaks two-step OTAs.

This CL adds update_bootloader_message(<options>), which only sets
the command field (to "boot-recovery") and the recovery field (with
the specified options).

Bug: 33534933
Test: Apply a two-step package.
Test: recovery_component_test passes.
Change-Id: Ie0b1ed4053d2d3c97d9cb84310d616b28fcfc72e
diff --git a/recovery.cpp b/recovery.cpp
index 0fdc31c..0da3946 100644
--- a/recovery.cpp
+++ b/recovery.cpp
@@ -354,20 +354,21 @@
   // bootloader control block. So the device will always boot into recovery to
   // finish the pending work, until finish_recovery() is called.
   std::vector<std::string> options(args.cbegin() + 1, args.cend());
-  if (!write_bootloader_message(options, &err)) {
-    LOG(ERROR) << err;
+  if (!update_bootloader_message(options, &err)) {
+    LOG(ERROR) << "Failed to set BCB message: " << err;
   }
 
   return args;
 }
 
-static void
-set_sdcard_update_bootloader_message() {
-    std::vector<std::string> options;
-    std::string err;
-    if (!write_bootloader_message(options, &err)) {
-        LOG(ERROR) << err;
-    }
+// Set the BCB to reboot back into recovery (it won't resume the install from
+// sdcard though).
+static void set_sdcard_update_bootloader_message() {
+  std::vector<std::string> options;
+  std::string err;
+  if (!update_bootloader_message(options, &err)) {
+    LOG(ERROR) << "Failed to set BCB message: " << err;
+  }
 }
 
 // Read from kernel log into buffer and write out to file.
@@ -485,7 +486,7 @@
     // Reset to normal system boot so recovery won't cycle indefinitely.
     std::string err;
     if (!clear_bootloader_message(&err)) {
-        LOG(ERROR) << err;
+        LOG(ERROR) << "Failed to clear BCB message: " << err;
     }
 
     // Remove the command file, so recovery won't repeat indefinitely.
@@ -1323,7 +1324,7 @@
     // Increment the retry counter by 1.
     options.push_back(android::base::StringPrintf("--retry_count=%d", retry_count+1));
     std::string err;
-    if (!write_bootloader_message(options, &err)) {
+    if (!update_bootloader_message(options, &err)) {
         LOG(ERROR) << err;
     }
 }