Add support for iSCSI boot

[rgl]

It would be pretty nice to have support for booting from an iSCSI disk.

This could be used as a quick way to test the iSCSI infrastructure.

This requires, at least, the iscsi_ibft module.

For reference, currently, this fails to find the rootfs as:

info: initramfs: up at 0.95
currently loaded modules: 8021q 8139cp 8390 9pnet 9pnet_virtio ahci cec drm drm_kms_helper e1000 e1000e failover fb_sys_fops garp hid hid_generic ip6_udp_tunnel ip_tables isofs libahci libcrc32c llc mii mrp ne2k_pci net_failover nls_ascii nls_iso8859_1 nls_utf8 pcnet32 qemu_fw_cfg rc_core sctp stp syscopyarea sysfillrect sysimgblt udp_tunnel usbhid virtio_blk virtio_dma_buf virtio_gpu virtio_input virtio_net virtio_rng virtio_scsi x_tables 
info: initramfs did not find root, running as ramdisk
info: /etc/init.d/rc.sysinit: up at 1.08
info: container: none
Starting syslogd: OK
currently loaded modules: 8021q 8139cp 8390 9pnet 9pnet_virtio ahci cec drm drm_kms_helper e1000 e1000e failover fb_sys_fops garp hid hid_generic ip6_udp_tunnel ip_tables isofs libahci libcrc32c llc mii mrp ne2k_pci net_failover nls_ascii nls_iso8859_1 nls_utf8 pcnet32 qemu_fw_cfg rc_core sctp stp syscopyarea sysfillrect sysimgblt udp_tunnel usbhid virtio_blk virtio_dma_buf virtio_gpu virtio_input virtio_net virtio_rng virtio_scsi x_tables 

[osfrickler]

If you propose a patch to add the module, github will create image artifacts for you that you can use to test this scenario. If you document how it works, we could consider adding it to the official release, then.