create-image.sh: avoid the use of package_type()

package_type won't work since $DISTRO global var is not defined here.
create-image.sh: adjust the extlinux.conf

use extlinux.conf syntax advised in the official documentation. This
was needed to get a RHEL boot.
replace FTP protocol with HTTP when possible

HTTP is more and more the sole protocol accepted by HTTP proxy.
doc: syslinux-tftpboot is in an extra RedHat repo

syslinux-tftpboot is not in the base repository on RedHat.
rhel-x86_64-server-optional-6.5.z has to be activated.
doc: fix the pxelinux example file

“APPEND” parameters have to be on the same line.