Setting a profile in a newly created sysroot when building native
toolchains broke after an eselect update. Apparently eselect gets the
path to the coreos-overlay repository and then prefixes it with
ROOT. Since ROOT was set to /build/<arch>-usr, the resulting patch was
wrong. Fix this by telling eselect where to find our make.profile
symlink in new sysroot by setting PORTAGE_CONFIGROOT to
/build/<arch>-usr and where to find our profiles by setting ROOT (and
SYSROOT, because it must match ROOT) to /.