release: create a bootable image with the graphical installer

[khorben_defora.org]

Summary

This commit adds an "installer" target to the amd64 release directory, generating a redistributable bootable image containing the graphical installer. This includes a graphical live environment, which currently defaults to the DeforaOS desktop environment. This should probably be changed for something lightweight enough while also more familiar to regular end users, such as XFCE.

The images currently generated are slightly below 8 GB in my tests.

This review depends on D44279, D44280, D44670, D44671, D44672, D44673, D44674.

Test Plan

To build:

src# make buildenv
Entering world for amd64:amd64
src# (cd release && make installer)
[...]

Possible way to test:

$ qemu-system-x86_64 -M type=q35,accel=hvf -cpu host -smp 2 -m 1024M -vga virtio \
        -drive if=none,id=usbstick,format=raw,file=installer.img \
        -usb \
        -device usb-ehci,id=ehci \
        -device usb-tablet,bus=usb-bus.0 \
        -device usb-storage,bus=ehci.0,drive=usbstick