Fix live boot: custom syslinux.cfg and explicit squashfs/overlay modules

The releng syslinux.cfg references UI .c32 modules that may not load,
causing syslinux to fall back without archisolabel= in the kernel cmdline
so the archiso initrd hook scans for an empty device label and sets root=''.

- Write our own minimal syslinux.cfg using %INSTALL_DIR%/%ARCHISO_LABEL%
  placeholders; Dockerfile now only copies .c32 binaries from releng
- Add squashfs and overlay to mkinitcpio MODULES explicitly
- Remove microcode hook (no intel-ucode/amd-ucode packages installed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/profile/airootfs/etc/mkinitcpio.conf

@@ -1,4 +1,4 @@
-MODULES=()
+MODULES=(squashfs overlay)
 BINARIES=()
 FILES=()
-HOOKS=(base udev microcode archiso block filesystems keyboard)
+HOOKS=(base udev archiso block filesystems keyboard)