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",
diff --git a/build/toolchain/embedded.gni b/build/toolchain/embedded.gni
index 6785701..fa26b2e 100644
--- a/build/toolchain/embedded.gni
+++ b/build/toolchain/embedded.gni
@@ -98,8 +98,6 @@
 template("embedded_clang_toolchain") {
   assert(defined(invoker.target),
          "\"target\" must be defined for ${target_name}.")
-  assert(defined(invoker.tool_prefix),
-         "\"tool_prefix\" must be defined for ${target_name}.")
   assert(defined(invoker.extra_defines),
          "\"extra_defines\" must be defined for ${target_name}")
   assert(defined(invoker.extra_cflags),
@@ -128,7 +126,6 @@
         " -O2 -lto-O2 --icf=all --fatal-warnings --color-diagnostics"
 
     toolchain_args = {
-      tool_prefix = invoker.tool_prefix
       if (defined(invoker.toolchain_args)) {
         forward_variables_from(invoker.toolchain_args, "*")
       }
@@ -166,7 +163,6 @@
     }
 
     toolchain_args = {
-      tool_prefix = invoker.tool_prefix
       if (defined(invoker.toolchain_args)) {
         forward_variables_from(invoker.toolchain_args, "*")
       }