Support UEFI PXE boot with DHCPv4

pxelinux supports booting via BIOS, but to get EFI boot working via PXE
(especially when booting via Secure Boot even), then we need a proper
GRUB based boot toolchain.

By using binaries from grub-efi-amd64-signed + shim-signed packages and
integrating them in our configuration, we support BIOS boot via pxelinux
and EFI boot via GRUB at the same time.

FTR: to support grml-terminalserver as arch all package and not
*strictly* requiring the grub-efi-amd64-signed + shim-signed packages
(at least for now), we use a dependency chain as follows:

| grub-efi-amd64-signed | grub-efi-amd64-bin | pxelinux,
| shim-signed:amd64 | pxelinux,

This means that only pxelinux is strictly needed if you don't care about
PXE support, but need to ensure grub-efi-amd64-signed (or
grub-efi-amd64-bin as its fallback) and shim-signed are present on your
system if you want PXE boot support within grml-terminalserver.  This
change will be implemented in GRML_FULL class of grml-live for the
official Grml ISOs.

This work was funded by Grml-Forensic.