Add archiso initramfs hooks so the live ISO can switch root

The profile shipped boot configs and the package list but lacked the
mkinitcpio archiso configuration, so mkarchiso built a stock initramfs
with no archiso hook. At boot the kernel honoured archisosearchuuid/
archisobasedir but nothing knew how to find and mount airootfs.sfs, so
switch-root failed and the live medium dropped to emergency mode.

Add the canonical releng pieces:
- airootfs/etc/mkinitcpio.conf.d/archiso.conf (HOOKS incl. archiso)
- airootfs/etc/mkinitcpio.d/linux.preset (builds initramfs-linux.img)
- mkinitcpio{,-archiso,-nfs-utils} in packages.x86_64

iso/airootfs/etc/mkinitcpio.conf.d/archiso.conf

@@ -0,0 +1,3 @@
+HOOKS=(base udev microcode modconf kms memdisk archiso archiso_loop_mnt archiso_pxe_common archiso_pxe_nbd archiso_pxe_http archiso_pxe_nfs block filesystems keyboard)
+COMPRESSION="xz"
+COMPRESSION_OPTIONS=(-9e)

iso/airootfs/etc/mkinitcpio.d/linux.preset

@@ -0,0 +1,8 @@
+# mkinitcpio preset file for the 'linux' package on archiso
+
+PRESETS=('archiso')
+
+ALL_kver='/boot/vmlinuz-linux'
+archiso_config='/etc/mkinitcpio.conf.d/archiso.conf'
+
+archiso_image="/boot/initramfs-linux.img"

iso/packages.x86_64

@@ -12,6 +12,12 @@ btrfs-progs
 dosfstools
 mtools
 # Live-ISO boot (archiso bootmodes: bios.syslinux + uefi.systemd-boot)
+# mkinitcpio-archiso provides the initramfs hooks that find and mount
+# airootfs.sfs and switch root into it — without it the live ISO drops
+# to emergency mode on boot.
+mkinitcpio
+mkinitcpio-archiso
+mkinitcpio-nfs-utils
 syslinux
 memtest86+
 memtest86+-efi