Add x86 network boot support to make_netboot.sh
Add x86 network boot support to make_netboot.sh

Current make_netboot.sh only supports ARM. As we are using network boot
for x86 now, let's fix make_netboot.sh to support both.

BUG=chrome-os-partner:9805
TEST=Generates images and network boot install shim.

Change-Id: Ib445f68255fe8e8a1ee6b7901c9bd67a4a36636d
Reviewed-on: https://gerrit.chromium.org/gerrit/23010
Reviewed-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Commit-Ready: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Tested-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>

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