How to pxe boot Assisted installer ISO image

gistfile1.txt

1) Download the discovery iso image, small size bootable is ok, no need to download the full image
2) mkdir -p /mnt/iso && mount -o ro,loop *iso /mnt/iso && mkdir -p /mnt/efiboot 
   && mount -o loop,ro /mnt/iso/images/efiboot.img /mnt/efiboot
3) optionally install web server if downloading from openshift mirror is slow. Assume the http root directory is /var/www/html, 
    curl -L -o /var/www/html/rhcos-live-rootfs.x86_64.img https://mirror.openshift.com/pub/openshift-v4/dependencies/rhcos/4.9/4.9.0/rhcos-live-rootfs.x86_64.img
4) install dnsmasq and tftp service. Assume tftp root is /var/lib/tftpboot
    /bin/cp -f /mnt/iso/images/*.img /var/lib/tftpboot
    /bin/cp -f /mnt/iso/images/pxeboot/{initrd.img,vmlinuz} /var/lib/tftpboot
    /bin/cp -f /mnt/efiboot/EFI/redhat/{shimx64.efi,grubx64.efi} /var/lib/tftpboot
cat <<EOF > /var/lib/tftpboot/grub.cfg
set timeout=1
menuentry 'Install Red Hat Enterprise Linux CoreOS' --class fedora --class gnu-linux --class gnu --class os {
	linuxefi vmlinuz random.trust_cpu=on ignition.firstboot ignition.platform.id=metal 'coreos.live.rootfs_url=https://mirror.openshift.com/pub/openshift-v4/dependencies/rhcos/4.9/4.9.0/rhcos-live-rootfs.x86_64.img' 
	initrdefi initrd.img ignition.img
}
EOF

5) sample dnsmasq setting
enable-tftp
tftp-root=/var/lib/tftpboot
dhcp-vendorclass=BIOS,PXEClient:Arch:00000
dhcp-boot=tag:BIOS,lpxelinux.0
dhcp-boot=tag:!BIOS,shimx64.efi

or simply enforce UEFI and not to support legacy boot,
enable-tftp
tftp-root=/var/lib/tftpboot
dhcp-boot=shimx64.efi

6) boot the server
pxe_opt="options=efiboot"
ipmitool -I lanplus -H ${ipmi_addr} -U ${ipmi_user} -P ${ipmi_password} chassis bootdev pxe ${pxe_opt}
ipmitool -I lanplus -H ${ipmi_addr} -U ${ipmi_user} -P ${ipmi_password} chassis power cycle