build_image: clean up emit_gpt_scripts a bit
build_image: clean up emit_gpt_scripts a bit

Looks like the func was copy & pasted, so delete the first one (which
doesn't get used).  Then expand on the existing func to also generate
a mount and an umount script.

BUG=None
TEST=ran build_image, then tested the mount/umount and unpack/pack scripts

Change-Id: I34a372c7b4858b8e9057a29b2eb58c38d547eadd
Reviewed-on: https://gerrit.chromium.org/gerrit/33929
Reviewed-by: Liam McLoughlin <a63c1d66dbd75d3cc434e11bdf376be6f9ff043b@chromium.org>
Tested-by: Liam McLoughlin <a63c1d66dbd75d3cc434e11bdf376be6f9ff043b@chromium.org>
Commit-Ready: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

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