make_chroot: fix make.conf.user generation

Make sure we don't expand variables that we intend to be literals.

BUG=chromium:1059101
TEST=None

Change-Id: Ic7564f734b1ff4a3f317dd2deb1b38ae7f70b1b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2089970
Reviewed-by: Dana Goyette <dgoyette@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
1 file changed