build_image_util: fix generation of packages.txt
build_image_util: fix generation of packages.txt

The passing ROOT= as an environment variable to board wrapper scripts
doesn't work, the script unconditionally overrides it. This means so far
our packages.txt files have listed the contents of /build/amd64-usr
instead of the image. Fix this by calling equery directly instead.

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