toolchain-utils: specify full path in script to be run in chroot.

Remove the assumption that the current directory is ~/trunk/src/scripts/
when the chroot is entered with cros_sdk.

BUG=chromium:429765
TEST=Ran image_chromeos.py with the changes.

Change-Id: I4aa9f233da5b3eef727009d044acdd1a27ea4873
Reviewed-on: https://chrome-internal-review.googlesource.com/219940
Reviewed-by: Caroline Tice <cmtice@google.com>
Commit-Queue: Rahul Chaudhry <rahulchaudhry@google.com>
Tested-by: Rahul Chaudhry <rahulchaudhry@google.com>
diff --git a/image_chromeos.py b/image_chromeos.py
index e9c581c..dd9a336 100755
--- a/image_chromeos.py
+++ b/image_chromeos.py
@@ -193,7 +193,7 @@
             chroot_image = located_image[len(real_chroot_dir):]
         else:
           chroot_image = os.path.join(
-              "..",
+              "~/trunk/src",
               located_image[len(real_src_dir):].lstrip("/"))
 
       # Check to see if cros flash will work for the remote machine.