Add Howto Update content

HowtoUpdate.md

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+TFTP server
+==
+
+HOWTO update
+--
+
+### Copy new images
+* Arch (static)
+  * Copy `arch` folder from the ISO in `images/archlinux`
+* Arch netboot
+  * We can also use the iPXE chainloaded image and boot from Internet
+  * But that means we download 500 MB from Internet...
+  * And that we need more than 1 GB of RAM
+* Ubuntu Live
+  * Copy all content from the ISO in `images/ubuntu-live/<arch>`
+* Ubuntu/Debian Installers
+  * Use `netboot.tar.gz` (from https://deb.debian.org/debian/dists/<version>/main/installer-<arch>/current/images/netboot/)
+  * Copy directly `debian-installer` to `images`
+
+### Upon update
+#### With GNU cp (Linux)
+* Remove `/srv/tftp/efi64/images` and create the hard link again (mind the `ubuntu` symlink in Ubuntu Live)
+#### With OpenBSD
+* apply changes in both `/srv/tftp/bios/images` and `/srv/tftp/efi64/images`