fix(vm_image_util): Fix building squashfs for /usr
fix(vm_image_util): Fix building squashfs for /usr

Taking a bit of a new approach to booting PXE images here for both
amd64-generic and amd64-usr. Instead of requiring the user to specify
squashfs and tmpfs on the kernel command line we can simply provide
defaults in the initrd's fstab.

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