Add configs/*/grub/loopback.cfg

See https://www.supergrubdisk.org/wiki/Loopback.cfg for details.

Only `${iso_path}` is guaranteed, so we need to search for the volume,
on which the ISO file resides, ourselves.

Implements https://gitlab.archlinux.org/archlinux/archiso/-/issues/165

grub/loopback.cfg

@@ -0,0 +1,80 @@
+# https://www.supergrubdisk.org/wiki/Loopback.cfg
+
+# Search for the ISO volume
+search --no-floppy --set=archiso_img_dev --file "${iso_path}"
+probe --set archiso_img_dev_uuid --fs-uuid "${archiso_img_dev}"
+
+# Get a human readable platform identifier
+if [ "${grub_platform}" == 'efi' ]; then
+    archiso_platform='UEFI'
+    if [ "${grub_cpu}" == 'x86_64' ]; then
+        archiso_platform="x64 ${archiso_platform}"
+    elif [ "${grub_cpu}" == 'i386' ]; then
+        archiso_platform="IA32 ${archiso_platform}"
+    else
+        archiso_platform="${grub_cpu} ${archiso_platform}"
+    fi
+elif [ "${grub_platform}" == 'pc' ]; then
+    archiso_platform='BIOS'
+else
+    archiso_platform="${grub_cpu} ${grub_platform}"
+fi
+
+# Set default menu entry
+default=archlinux
+timeout=15
+timeout_style=menu
+
+
+# Menu entries
+
+menuentry "Arch Linux install medium (%ARCH%, ${archiso_platform})" --class arch --class gnu-linux --class gnu --class os --id 'archlinux' {
+    set gfxpayload=keep
+    linux /%INSTALL_DIR%/boot/%ARCH%/vmlinuz-linux archisobasedir=%INSTALL_DIR% img_dev=UUID=${archiso_img_dev_uuid} img_loop="${iso_path}"
+    initrd /%INSTALL_DIR%/boot/intel-ucode.img /%INSTALL_DIR%/boot/amd-ucode.img /%INSTALL_DIR%/boot/%ARCH%/initramfs-linux.img
+}
+
+menuentry "Arch Linux install medium with speakup screen reader (%ARCH%, ${archiso_platform})" --hotkey s --class arch --class gnu-linux --class gnu --class os --id 'archlinux-accessibility' {
+    set gfxpayload=keep
+    linux /%INSTALL_DIR%/boot/%ARCH%/vmlinuz-linux archisobasedir=%INSTALL_DIR% img_dev=UUID=${archiso_img_dev_uuid} img_loop="${iso_path}" accessibility=on
+    initrd /%INSTALL_DIR%/boot/intel-ucode.img /%INSTALL_DIR%/boot/amd-ucode.img /%INSTALL_DIR%/boot/%ARCH%/initramfs-linux.img
+}
+
+
+if [ "${grub_platform}" == 'efi' -a "${grub_cpu}" == 'x86_64' -a -f '/boot/memtest86+/memtest.efi' ]; then
+    menuentry 'Run Memtest86+ (RAM test)' --class memtest86 --class memtest --class gnu --class tool {
+        set gfxpayload=800x600,1024x768
+        linux /boot/memtest86+/memtest.efi
+    }
+fi
+if [ "${grub_platform}" == 'pc' -a -f '/boot/memtest86+/memtest' ]; then
+    menuentry 'Run Memtest86+ (RAM test)' --class memtest86 --class memtest --class gnu --class tool {
+        set gfxpayload=800x600,1024x768
+        linux /boot/memtest86+/memtest
+    }
+fi
+if [ "${grub_platform}" == 'efi' ]; then
+    if [ "${grub_cpu}" == 'x86_64' -a -f '/shellx64.efi' ]; then
+        menuentry 'UEFI Shell' --class efi {
+            chainloader /shellx64.efi
+        }
+    elif [ "${grub_cpu}" == "i386" -a -f '/shellia32.efi' ]; then
+        menuentry 'UEFI Shell' --class efi {
+            chainloader /shellia32.efi
+        }
+    fi
+
+    menuentry 'UEFI Firmware Settings' --id 'uefi-firmware' {
+        fwsetup
+    }
+fi
+
+menuentry 'System shutdown' --class shutdown --class poweroff {
+    echo 'System shutting down...'
+    halt
+}
+
+menuentry 'System restart' --class reboot --class restart {
+    echo 'System rebooting...'
+    reboot
+}