Use LLVM objcopy from prebuilts rather than system version.

Bug: 132428451
Change-Id: I519d846fabdf6a45aced924ef50e61d9dcb7cee7
diff --git a/build/image/convert_to_binary.py b/build/image/convert_to_binary.py
index bfbc609..482b70e 100644
--- a/build/image/convert_to_binary.py
+++ b/build/image/convert_to_binary.py
@@ -21,19 +21,21 @@
 """
 
 import argparse
+import os
 import subprocess
 import sys
 
+HF_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(__file__)))
+CLANG_ROOT = os.path.join(HF_ROOT, "prebuilts", "linux-x64", "clang")
+OBJCOPY = os.path.join(CLANG_ROOT, "bin", "llvm-objcopy")
 
 def Main():
     parser = argparse.ArgumentParser()
-    parser.add_argument("--tool_prefix", required=True)
     parser.add_argument("--input", required=True)
     parser.add_argument("--output", required=True)
     args = parser.parse_args()
     subprocess.check_call([
-        "{}objcopy".format(args.tool_prefix), "-O", "binary", args.input,
-        args.output
+        OBJCOPY, "-O", "binary", args.input, args.output
     ])
     return 0
 
diff --git a/build/image/image.gni b/build/image/image.gni
index ebde0c2..46b5af2 100644
--- a/build/image/image.gni
+++ b/build/image/image.gni
@@ -62,8 +62,6 @@
       ":${target_name}__elf",
     ]
     args = [
-      "--tool_prefix",
-      tool_prefix,
       "--input",
       rebase_path(elf_file),
       "--output",