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/syslinux/syslinux.cfg

@@ -0,0 +1,8 @@
+DEFAULT arch64
+TIMEOUT 50
+PROMPT 0
+
+LABEL arch64
+    LINUX /%INSTALL_DIR%/boot/x86_64/vmlinuz-linux
+    INITRD /%INSTALL_DIR%/boot/x86_64/initramfs-linux.img
+    APPEND archisobasedir=%INSTALL_DIR% archisolabel=%ARCHISO_LABEL% rw