bootloader: align with AOSP code

- Define a minimal Volume structure so that AOSP code for handling /misc
  works in TWRP without major changes.
- Remove set/get_bootloader_message_*_name

Change-Id: Ifcee59568141a184adebb94b8ef8beca072a7942
diff --git a/twrp-functions.cpp b/twrp-functions.cpp
index b6f9c56..3bc8799 100644
--- a/twrp-functions.cpp
+++ b/twrp-functions.cpp
@@ -487,15 +487,8 @@
 	if (Part != NULL) {
 		struct bootloader_message boot;
 		memset(&boot, 0, sizeof(boot));
-		if (Part->Current_File_System == "mtd") {
-			if (set_bootloader_message_mtd_name(&boot, Part->MTD_Name.c_str()) != 0)
-				LOGERR("Unable to set MTD bootloader message.\n");
-		} else if (Part->Current_File_System == "emmc") {
-			if (set_bootloader_message_block_name(&boot, Part->Actual_Block_Device.c_str()) != 0)
-				LOGERR("Unable to set emmc bootloader message.\n");
-		} else {
-			LOGERR("Unknown file system for /misc: '%s'\n", Part->Current_File_System.c_str());
-		}
+		if (set_bootloader_message(&boot) != 0)
+			LOGERR("Unable to set bootloader message.\n");
 	}
 
 	if (PartitionManager.Mount_By_Path("/cache", true)) {