arm64 graphical installer fixes

Some small fixes that I've already posted (except for the last one) to the mailing lists a while back, scattered in this thread about the graphical installer and bug #961590.

On arm64, tty0 isn't the preferred console if the device-tree includes a chosen stdout-path property; and it isn't even in the /proc/consoles list if the device has an ACPI SPCR table. A rootskel patch considers tty0 a possible console even if it's not in /proc/consoles. The netboot installer already launches on the display in the stdout-path case, but not on the SPCR case. With this it launches on the display in that case as well.

The first commit here adds "console=tty0" to GRUB entries marked as "Graphical" in grub-gencfg. Without this, "Graphical automated install" runs on the serial console (since that may be the "preferred" console due to stdout-path or SPCR on arm64). The "console=tty0" cmdline argument also ends up in the installed system's /etc/default/grub.cfg, which is correct IMO.

The second commit fixes arm64 netboot and netboot-gtk mini.iso images which currently have 'Graphical install' GRUB entries using /gtk/initrd.gz files which don't exist, by making that file identical to /initrd.gz.

After those commits, on an arm64 QEMU virtual machine:

• "Install" launches on serial (preferred) and display
• "Graphical install" launches on display (preferred) and serial
• "Automated install" launches only on serial
• "Graphical automated install" launches only on display

On systems where console is set with stdout-path instead, "Graphical install" would launch only on the display, but the others would be the same.

The third commit includes fb-modules in the non-gtk cdrom image to let a text-mode installer launch on the display as reported in bug #961590.

The fourth and last commit generates concatenateable SD-card images for arm netboot-gtk just like arm netboot does.