Fix broken boto file handling.
Fix broken boto file handling.

After CL:39921, I get the following warning every time I enter the chroot:

ln: failed to create symbolic link `.../chroot/root/.boto': File exists

All bots get this error as well. This is caused because CL:39921, causes
~/trunk to no longer resolve outside the chroot, so it's invalid for processes
outside the chroot to try to resolve paths inside there. Fix cases where we do
this inside enter_chroot.sh.

BUG=chromium-os:37347
TEST=cros_sdk doesn't print warnings anymore.

Change-Id: Iaeb9b7407e12397bce1600bd51559be20f998fdf
Reviewed-on: https://gerrit.chromium.org/gerrit/41571
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Queue: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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