cgpt_shell: avoid bash
cgpt_shell: avoid bash

This script gets run on the target, so we need to avoid bash.

BUG=chromium-os:34715
TEST=build_image works

Change-Id: Ib64036246149b8b98414e4434ed89a3a40c2f693
Reviewed-on: https://gerrit.chromium.org/gerrit/33911
Reviewed-by: Liam McLoughlin <a63c1d66dbd75d3cc434e11bdf376be6f9ff043b@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

BugRoger/scripts,zhang0137/scripts,marineam/coreos-scripts,fivethreeo/scripts,trnubo/scripts,nicescale/scripts,fivethreeo/scripts,bcwaldon/coreos-scripts,smilart/scripts,glevand/coreos--scripts,fivethreeo/scripts,mjg59/scripts,bcwaldon/coreos-scripts,andrejro/coreos--scripts,endocode/scripts,gtank/scripts,trnubo/scripts,coreos/scripts,zhang0137/scripts,BugRoger/scripts,gtank/scripts,sigma/coreos-scripts,trnubo/scripts,zhang0137/scripts,andrejro/coreos--scripts,vmware/coreos-scripts,sigma/coreos-scripts,smilart/scripts,endocode/scripts,BugRoger/scripts,nicescale/scripts,vmware/coreos-scripts,coreos/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,sigma/coreos-scripts,crawford/scripts,mjg59/scripts,mischief/scripts,mjg59/scripts,glevand/coreos--scripts,mischief/scripts,andrejro/coreos--scripts,marineam/coreos-scripts,gtank/scripts,mischief/scripts,crawford/scripts,smilart/scripts,endocode/scripts,nicescale/scripts,marineam/coreos-scripts,crawford/scripts