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/common.h b/common.h
index 18b0304..c6300e8 100644
--- a/common.h
+++ b/common.h
@@ -45,7 +45,7 @@
 #define STRINGIFY(x) #x
 #define EXPAND(x) STRINGIFY(x)
-typedef struct fstab_rec Volume;
+//typedef struct fstab_rec Volume;
 // fopen a file, mounting volumes and making parent dirs as necessary.
 FILE* fopen_path(const char *path, const char *mode);