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>
1 file changed