build/pxe.install: only add the rhel-x86_64-server-sfs-6 under RHEL6
build/pxe.install: only add the rhel-x86_64-server-sfs-6 under RHEL6

enovance/edeploy,enovance/edeploy,lebauce/edeploy,lebauce/edeploy,redhat-cip/edeploy,enovance/edeploy,redhat-cip/edeploy,lebauce/edeploy,lebauce/edeploy,redhat-cip/edeploy,enovance/edeploy,redhat-cip/edeploy