create-image.sh: disk image generation now supports grub1 based distribution

Add grub1 support when generating virtual disk image. However we use extlinux
as installing grub1 in a chroot is not very reliable.

During the virtual disk image creation the bootloader installation process
detect whether grub2 is available or not, if not extlinux is used.
create-image.sh: basic network config for redhat based linux

When generating virtual disk image for redhat based linux, the automatic
network configuration settings is now properly handled. ndp is also
functionnal.