build_image: do not use gs:// URLs in dev images
build_image: do not use gs:// URLs in dev images

When building dev images, the PORTAGE_BINHOST value during build
time is written to the image's make.conf.  This breaks the default
binary package setup, since Jenkins is using gs:// URLs for signed
package verification and authenticated downloads, and the make.conf
doesn't inherit the GS_* variables to handle those schemes.

This should be reverted when signed packages are properly supported
by default in the dev images.

coreos/scripts,glevand/coreos--scripts,glevand/coreos--scripts,coreos/scripts